Here are the items you can use. You need to be playing a game where Items are allowed though in order to get them.
Green Shell- Throw a Green Shell and knock down your opponents.
Red Shell- Throw a Red Shell that will chase after the nearest opponent and knock them down.
Blue Shell- Throw a Blue Shell to freeze your opponent.
Spiny Shell- Throw out Spiny Shells that bouce around the field, knocking down anything in their path.
Banana- Drop a well-placed banana peel and watch your opponents slip.
Bob-omb- Launch a Bob-omb and watch your opponents fly across the screen from the blast.
Chain Chomp- Set a Chain Chomp loose on the field and disable your opponent's entire team.
Mushroom- Provides a limited burst of super speed.
Star- Become invincible for a short time. Any opponent who touches you will be knocked aside!