Because of the speed of the game, sometimes you might get a little flustered about which card to choose. You can counteract that by getting to know the decks you're going to be using before you head out into a battle. You'll unlock the starter decks by defeating them, and once you have it's well worth jumping into the deck editor and finding out what's what. It's also a good idea to play some practice matches against the AI to find out which deck suits your play style best.