At the beginning of your upkeep, reveal the top card of your library. If it's a creature card that shares a creature type with a creature you control, you may cast that card without paying its mana cost. Otherwise, put that card on the bottom of your library.
5/1/2012 If you cast a card “without paying its mana cost,” you can’t pay any alternative costs. You can pay additional costs, such as kicker costs. If the card has mandatory additional costs, you must pay those.
5/1/2012 If the card has in its mana cost, you must choose 0 as its value.
5/1/2012 If the revealed card is a creature card that shares a creature type with a creature you control, but you choose not to cast it, it’s put on the bottom of your library. (This is a recent change.)