my macbook pro is extremely slow

My macbook pro has gotten extremely slow and every thing I try to do I get the spinning color wheel. After I close the lid and when I re open it it takes forever to load. The wheel just spins and spins. What do I need to do to correct this problem. I am running mountain lion 10.8.2. How do What are things I can do to clean up the computer and make it run more effeciently.

thanks

Glenda