I think you have to play something, even if it won't work. In such a case, the damage you deal that turn is 0. I imagine it isn't that great, but hopefully next turn you'll draw something better. If you consistently keep getting cards that won't work, then it might be a sign that you need to fix something with either your creatures or your attacks (in the future, I mean, when you actually start building decks).
In girum imus nocte et consumimur igni