How to defragment a hard drive is easy, but first I'll explain what a computer does when it degraments. When you defragment your hard drive, your are just moving little file pieces that have been written all over you hard drive into a one solid piece. This makes the file faster to read back since it's located in 1 location instead of 10.
The benefit of doing a defragment weekly is simple, since a file is now in one place instead of ten, it if faster for your computer to find all the data of that file. The result, your computer is faster and loading times are reduced.
It may take a couple of seconds to load, it is reading the current state of your hard drives.
That's it, it will take anywhere from 15 minutes to 8 hours to run. Once it's finished it will come up with a dialog telling you it's complete.