Macs are less affected by disk fragmentation than Windoze boxes. In general, you can expect a small speed boost by defragging once every year or two. Many Mac users never defrag.
Apple does not supply a defrag utility. You have to buy a 3rd-party utility such as PlusOptimizer (Alsoft - OS 9 only) or TechTool Pro 3 / 4 (Micromat - TTP3 is OS 9 only).
If you have at least two partitions (or at least two hard drives), you can copy across the entire partition / HD. This will defrag the disk also.