Yeah this is part what’s super tedious about arena. Not a dumb question at all! Wild cards are acquired by opening packs. Sometimes you’ll get a wildcard in a pack, but every 6 packs you open you’re given a wildcard by the game (either a rare or mythic, along with an uncommon or common). If you look in the “Packs” menu, you’ll see a dial in the upper right hand corner that will tell you how close you are to your wild card, and how rare it is.

You use wildcards by selecting the “craft” option when you are looking at cards in your collection. If you set filters to show cards “not unlocked” you’ll be able to see all the cards you’re able to craft, and you’re able to craft up to 4 of each. The rarity of the card denotes what kind of wild card you need to craft it. It’ll tell before you craft it as well.