You don’t.
There’s no need to. The OS X filesystem is designed to look after files properly in the first place, so that’s one thing you don’t have to worry about.
Sometimes – not often, but sometimes – you might hear your computer’s hard disk whirring for no apparent reason. Among other things, that could be the system looking after itself – moving stuff around on the disk so that there’s no need for you to actually sit down and click a button marked “defrag.”
If you want to know more about the technical ins-and-outs behind this, go read this Apple support document. As it points out, there’s no need to defrag your disk, and even if you download a third party defragging application and run it, you probably won’t notice any difference.
Save yourself the trouble, and spend your not-defragging time doing something fun on your computer instead.
(You’re reading the 28th post in our series, 100 Essential Mac Tips And Tricks For Windows Switchers. These posts explain to OS X beginners some of the most basic and fundamental concepts of using a Mac. Find out more.)
