So I picked up with Gwent pretty late into the game. So I lost track of where I was supposed to build my deck as a novice, almost every merchant I find beats me, unless I get really lucky.
As a person who spent a lot of time in Gwent and has all faction decks I would suggest you to do the following:
You can buy cards in taverns mostly hover look for them at every merchant(most of them are not that good but not bad to begin with) then i would suggest you to play with every merchant alchemist etc, cos they are not that strong as the special card holders.
Begin to play with Foltest who is able to make good weather as your first deck will be Northern Realms and always keep minimum 22 actually ammount of cards in your deck(you have to leave only the best cards and dont put rubbish into it) that is how you will be able to get your best cards with higher chance in those 10 that you get in the beginning of the game.
Don't hesitate to use training doll cards because it's pretty handy(especially to get enemy spy in your hand if you havn't got your own) but don't put way too much weather cards in your deck basically 1or2 will be enough and in case that you have special ability of Foltest you don't need "Good weather card"
If you want more advice you can send me private message. Atm i play for Nilfgaard and have 6 heroes in my deck and i don't have any card in my lower then 5 points.
P.S you don't need to play more then once to get cards.
P.P.S If you have only stock Foltest card you should not put into your deck mist cos your leader ability will cast it.