Duff-Man says...please...try searching the forums....a simple search using the term "defrag" would give you seveeral hits that would answer this and also show you just how many times this gets asked..... .....oh yeah!
Currently with Mac OS X if your leave you Mac on 24/7 it will run daily, weekly, and monthly maintenance automatically. If you don't have it on 24/7, then you could look at Optimizer X.