Here ya go!
Personally I think the default difficulty setting is a bit shite. It's easy enough to allow you to ignore most of your cool witcher abilities like oils, signs and decoctions, but not so easy that you can just faceroll everything and focus on the story. Personally on my first playthrough I had it on 'Blood and Broken Bones' which was much more enjoyable, so I'd recommend either turning the difficulty up a notch or down a notch from the default, depending on how you find the combat.
Gwent has its own separate difficulty slider in the options menu.
Once you have made a potion or decoction or bomb once, you have it forever. As long as you have hard alcohol on you, ie alcohest or Dwarven spirit, each time you meditate you replenish all your stuff. So if you've used all your Swallow potions or Samum bombs, just meditate for an hour.
Bombs are used for destroying monster nests and are also really handy for bandit camps with lots of mobs grouped together. Chuck them with R1.
Once you start making oils with alchemy, always apply them to your sword before fighting the appropriate enemy type. Necrophage Oil and Hanged Man's Venom are especially useful early on. Your bestiary will tell you what oil something needs if you can't remember.
The wheel to choose Signs etc is on L1 (you also use this wheel for switching the crossbow, bombs, and special equippable items). Signs will deplete your stamina (yellow) bar. You generally want to keep Quen up all the time before and during fights, as it's like a shield. Axii is useful on humanoids to stun them (especially good against bandits with shields). Aard is handy for knocking bandits off horses or blowing down decrepit doors to get inside. Ignii is good in all fights, especially against drowners. Yrden is mostly used against wraiths.
When you level up or find a 'Place of Power' you get ability points. Particularly useful skills to use them on early on are Cat/Griffin School Techniques, Gourmet (if you find you're struggling HP-wise), Delusion, and either Muscle Memory or Strength Training depending on whether you favour fast or strong attacks. Once you have learnt a skill you have to equip it on the right (try to stick to one armor type across all slots (light or medium), and swap Cat/Griffin Techniques as your equipped skill accordingly). You unlock more slots as you level up.
Loot everything!!! Look for lootable objects in towns and villages (be careful of guards - it may be easier at night), and in people's houses, using your witcher senses. Pick up all the shit swords and blackjacks that bandits drop and sell them to blacksmiths or armorers (they will pay more than other vendors).
Once you've done his quest in White Orchard, Willis the dwarf can upgrade your chest armour for you (when you have the materials). If you get all the '?' on the White Orchard map and loot as you go, you should end up with plans for both Viper swords (steel and silver) plus all the necessary materials. The Nilfgaardian quartermaster in the top-left of the map can craft them for you, and they will last quite a while.
When choosing armour, resistances are much more important than the actual armour rating. Slashing, piercing and bludgeoning help vs humans, and elemental resistance helps vs wraiths, barghests and (surprise) elementals.
Once you are level 4 and have the money (about 600 crowns), the Temerian Armour set from the merchant outside White Orchard is a good upgrade until you can get the Griffin School armour at level 11. It is only light armour, but has good resistances. Don't forget to swap in Cat School Techniques as an active skill if you had been using Griffin School with your starter armour.
When the main quest sends you to see the Bloody Baron, just west of the bridge to his keep is a horse racing quest where you can win saddlebags to increase your carrying capacity.
Generally don't dismantle weapons at a blacksmith unless you get rare materials back like dimeritium. You'll usually be better off selling weapons/armour, and dismantling your other random crap like candelabras, hides and platters to get materials instead. Once you have access to Novigrad, the blacksmith and armorer there will give you the best prices early on for weapons/armour.
Much later in the game, you'll want to use the Master armorer in Velen to do your dismantling, as they return two of each component instead of one. This also applies to the Master/Grandmaster smiths in Beauclair, but they are more expensive.
Jewellery can often be dismantled to get gems, which sell for more than the jewellery itself did. Shells can be dismantled to get pearls which sell for a lot.
If you have good runes in a sword you can dismantle it to get the runes back and put them in your new sword. The rune that inflicts bleeding is particularly useful early on.
When you first get to Novigrad, find Vimme Vivaldi in Hierarch Square. Get a loan off him, and get him to change your Orens and Florens into Crowns. You can also try to beat him at Gwent for a unique card (don't do the follow-up opponent until after you've met Sigi Reuven on the main questline, as it can bug out).
Also off Hierarch Square is a merchant in a book shop. Sell all your books to him as once you've read them, they generally have no purpose.
When you first go to Skellige, look around the beach carefully and you can get your captain's fee back. Kill the sirens nearby, then the nest on the boat and the Ekhidna. If you get a mutagen from it you can make the Ekhidna Decoction which is really useful early on.
Probably best to go with a Northern Realms deck on a first playthrough as it's the most accessible and arguably the most powerful.
Basically it's best out of 3 rounds, the player with the most points on the board wins the round. Sometimes it's sensible to pass a round and save cards for later. Pay attention to how many cards your opponent has left!
There are lots of different types of cards:
Decoy - swap with any card on your side of the board. Generally you use these on an opponent's Spy card so you can play it straight back at them. You can also swap it with cards that have a special effect when played (ie Medics, or the Villentretenmerth card) to allow you to re-use that effect. Or you can just use it to stall and make your opponent use more cards if you're planning on passing.
Commander's Horn - these double the value of all non-Hero cards on the row you apply it to. The leader card you get from the guy in Vizima acts as a free Horn for your siege row (it does not stack with the normal Commander's Horn card).
Scorch - this kills the highest value non-Hero card(s) on the board.. You can scorch your own cards, which can sometimes still be worthwhile (for instance the highest non-Hero cards in play are worth 10, your opponent has three of them but you only have one. Or for another example, you want to play Villentretenmerth twice in a round - you have no Decoys, but you have a Medic. You could play him, Scorch him to get him into your discard pile, then replay him using the Medic).
Weather cards - these reduce the strength of all (non-Hero) cards on a row to 1, for both players. The Biting Frost card is particularly useful against opponents with a Monsters deck, which tends to have lots of front row Muster cards. The Clear Weather card (or the Foltest leader card bought in White Orchard) will get rid of all weather effects.
OTHER NOTABLE CARDS
Spies (eyeball icon) - these go on the opponent's side of the board and count towards their score, so the lower the number, the better. However when you play a Spy, you draw two new cards from your deck into your hand, which is extremely useful. Generally you play your spies at the start of the game (after Muster cards, if you have any). If your opponent uses a Spy against you, try to nick it with a Decoy card and use it back on them!
Hero cards - these cards have a white number on a black background and a little visual flourish when played. They tend to be quite high value and are not affected by Scorch, Weather, or Commander's Horn cards. Some also have secondary effects, i.e. Yennefer is also a Medic.
Medics (cross inside a heart icon) - these retrieve one (non-Hero, non-Special) card from your discard pile and instantly play it (you can check through your discard pile at any time by moving over it and pressing I think R2). Generally they are only used in rounds 2 or 3, as in round 1 you won't have a discard pile unless something got scorched. Generally your first priority for revival will be another medic, which you can then use to bring back something else (perhaps a Spy that you weren't able to retrieve with Decoy).
Tight Bond cards (handshake icon) - when played with a matching card, both cards double in value (or triple, if you have three of them in your hand). If you then use a Commander's Horn on them, that's a lot of points (although they will be susceptible to Scorch!)
Morale Boost cards (cross icon) - these give +1 to all cards on their row, excluding themselves
Muster cards (helmets icon) - when played, they also play all other matching cards from your deck instantly. Having more than one matching Muster card in your hand is a waste, so if your starting hand contains two or more, swap them out using the re-draw you get before the game starts. Play your Muster cards before you use any Spies. The Monsters deck uses lots of Muster cards.
Buy all the cards from the innkeeper in White Orchard, then open up the start menu and check your deck. The cards on the left are ones you own but are not in your current deck, which is on the right. Move the cards you just bought into your deck, and change your leader card to the Clear Weather one. Generally speaking you want to keep your deck as close to 22 basic cards as possible at all times, so remove some of the weaker ones from your deck if desired (ie Redanian Foot Soldier). I also personally remove one of each weather card from my starting deck and just keep one of each instead of two. As you get stronger cards you can remove the weaker ones to keep as close to 22 basics as possible. Filtering out the weaker cards increases the chances of you drawing your stronger ones in your initial hand.
Now play the guy in the inn (Aldert Geert). You are allocated 10 random cards from your deck, and before the game you can re-draw up to 2 of them, if you want to. If you beat him you will get the Zoltan card. Add it to your deck. Don't forget you can play your new leader card with square to cancel out any weather effects, so if the opponent puts down a weather card and knocks loads of your cards down to 1, you can wait till near the end of the round then clear it with your leader, restoring your cards to their normal score.
After White Orchard if you didn't get those cards initially, you can go back and get them from the merchant outside the village, and the Zoltan card can be found at Hanged Man's Tree in Velen.
When you get to Vizima you can play a guy in the courtyard on your way out, he has a good deck but is beatable. Just bet 1 crown each time till you get him. You win another leader card (free Commander's Horn for your siege row), which you can change to your active leader if desired, or you can keep the one you bought in White Orchard.
Play everyone you can - you'll get a random card the first time you beat each vendor. There are also several Gwent quests you'll pick up to play specific people and win unique cards.
Always check innkeepers' stock to see if they sell any cards you can use. When you get to the village of Midcopse on the main quest, there's a merchant who sells some good Tight Bond ones. When you get to Skellige, the Arinbjorn innkeeper sells the very useful Thaler card. There's a merchant near Carsten that sells a couple of Muster cards.
Focus on buying Northern Realms or Neutral cards early on, but when you have more money feel free to start buying cards for other decks. You may even want to switch deck to Nilfgaard or Monsters at some point.
As you get more advanced you can start tailoring your deck to suit the opponent - for instance you may want to remove Decoys against someone using a Monsters deck, as that deck doesn't use Spy cards, and add in Biting Frost cards to counteract the front row Muster card spam.
As your deck gets more powerful the game can become easy, so you can put the difficulty up in the menu. The 'High Stakes' quest can be done towards the end of the game and needs a very strong deck.