I would recommend seeing some videos from streammers that made beginners guides, like chaoticpriest / https://www.youtube.com/user/Sarrok and KingBlackToof / https://www.youtube.com/user/KingBlackToof

I would also suggest playing with starter decks of each faction in casual so you can see what suits you better before commiting into crafting cards for an specific faction or buying kegs.

Also, play the tutorial to learn what has improven from the basic Witcher 3 Gwent, and iirc there is a card reward for finishing it too. There is new mechanics and cards abilities that are introduced in the tutorial and for simple as it is its still information and a chance to get used to it faster and not get caught off guard, right?

Take a look at the faction passives (for example, Northen Realms passive is that each gold card played gain 2 str; Monster is that the last card you played stays for the next round, and so on); when playing, everytime a card is played take a look at its description and try to understand what happened/what will happen.

Other than that there is some resource websites, like http://gwentlemen.com and http://www.gwentdb.com, where there is arcticles, lists of decks and things like that.

Everything else you'll learn from experience and playing. What are the current meta decks, what is strong at the moment, and so on. It can take a while to get used to it, but as you played Gwent in W3 it shouldn't be a problem for you.