When you start Prank Deck, you will be greeted with a random prank. If it is no good, simply swipe or flick the card in any direction to throw it away and get another random one.
You can mark any prank as a favorite by tapping the star in the upper left corner. Once you have some favorites, you can tap the bottom scroll button and check the "Show only favorites" box to just get your favorite pranks.
If you find a prank that you really love (or that worked really well), why not share it with your friends? Tap the top right share button to share a prank by e-mail or to post it on Facebook.