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.