Here are a few things that you will need.
Windows XP Home/Pro Installation Disk
Appropriate Drivers for all of your peripherals
(Display, Sound, Network, Modem, Printer, etc.)
A little common sense.
Once you have what you need, just reboot your computer with the Windows XP CD in it. It should ask you if you want to boot from CD-ROM, hit any key to continue, just follow the simple instructions after that.
Also, when you get to the screen that asks where you want to install XP, you might want to delete the partition, and recreate a new one. Afterwards format it by choosing "Format with NTFS (Quick)" Again, follow the on screen instructions and you will be ok.