There's no defragment needed under OS X.
You can run Disc Utility to repair permissions which can improve performance, and also "repair drive" which can correct other issues.
Disk Utility is located in the Utilities folder within Applications. Sometimes though, it will need to be run of the OS X CD/DVD if the issues need to be resolved from outside the OS. In that case, you'd put the disc in the computer, and reboot, holding the C key while the computer is booting to load the CD/DVD drive and following prompts.