One option is to drop several hundred euros into the game. That'll catch you right up.
If not...
Make a Renounce warlock. That one needs just 2 (literally two) cards - the two copies of Renounce Darkness - and its effectiveness is identical no matter how many cards you may or may not have in your collection. You literally just fill up your deck with any warlock cards, and that's it. Sure, you won't climb too high up the ranks, since it's not a serious deck, it's a shits'n'giggles deck. But it does several things for you:
- you get to win and do many of your quests
- you get to see and play hundreds of cards you'll otherwise never get to use, or even see
- you'll learn lateral thinking and strategy, you'll discover synergies and cards around which you may later build something serious
- you'll have ridiculous rewarding plays from time to time that just feel great (corollary: often you'll just completely flop, ofc)
The other way to do it is build one of the cheap easy variants of face hunter or zoo (hundreds of these to be found on hearthpwn, some require not much more than basic cards to be bad but usable) and just play that over and over and over again until you grind your way to a collection. Caveat: the boredom of doing that is soul crushing. You will look upon the shrivelled husk of the person you once were and ask yourself whyyyyyy.
So... yeah. For sanity's sake, Renounce.