On your turn you can:
-Play the next card up on the discard pile in sequence (Ace low, Queen high, Aces can be played on Queens)
-Play a card of the same value (a four on a four, a nine on a nine, etc)
-Play a King (the wildcard) and pick which suit your opponent must play next. If the game starts with a King, you can play any card of the same suit.
If you don't have any cards you can play you must keep drawing cards from the deck until you can make a play.