How to do defragment disk on Mac?


Please help, I'm a dummy for Mac. This is my first time that I use Mac System. I've tried "help" on tool bar and even bought a book "Mac for Dummies" but I could not find any help regarding to "Disk Defragment". If any members know how to get Disk defragment work or locate its icon please show me how to do or get it. Thank you very much.
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Another exceptionally useful Mac utility is MainMenu. It runs all the routine maintenance scripts that Albert mentions plus lots of other useful tasks like rebuilding your Spotlight index, repairing disk permissions, clearing caches and histories and removing temporary files, all from a single icon in the menu bar.
First of all, I would like to appreciate all members' information and knowledge. Second, I wanted to say thank all very much. Thanks again gentlemen.