It's pretty easy! After finishing the tutorial you get 5 starter decks, one for every colour, then a couple weeks playing matches unlocks 10 more structure decks, one for for each colour pair.
The booster packs look kinda pricey at first but I quickly realised you rack up SO much of the game's currency from the daily quests, which also sometimes offer packs as rewards. I'd say if an MTG noob like me can build a half decent deck by just cobbling together cards and seeing what sticks, then anyone more familiar with the game will probably find it to be a piece of cake.