Ascension is hands down the most straightforward gateway deckbuilding game there is. If you are interested in learning how this category of games works, start here! Ascension provides you with a row of cards to purchase and monsters to defeat, without the need to build an overwhelming strategy before beginning play (ahem, Dominion) or without having other game effects that you need to keep track of while creating your master deck (ahem, Legendary).

With something like twenty million expansions and editions, Ascension plays in about an hour and will absolutely help you learn how to build-a-deck.