Sarah,
There are many ways to do this.
1. Partition Magic. Great product. Problem=Not free.
2. Windows boot disk. Run fdisk from DOS. Problem=Have to delete the XP partition to create any new ones.
3. Most Linux distros will allow you to resize an existing partition during the install process. WinXP can either use FAT32 or NTFS. FAT32 is no problem, NTFS is usually read only by most Linux distros out of the box. Problem=I can't positively attest to this, I have never tried it.
4. Buy another hard drive. Installation is very simple. Problem=Again not free and you may not have installed a harddrive before.
5. If you are hoping to test drive Linux, try something like Knoppix (knopper.net). It runs completely from cd, no need to install.
Whatever you choose, be sure to backup important data first. I'd hate to see your next post in data recovery.
Good luck,
Mark