How To Partition A Disk
Partitioning a disk isn't always easy, and you really need to be aware of what you are doing - you will loose ALL information on a disk on a PC. So DO NOT try it unless you are really sure it is what you want to do, and are aware you are wiping your disk clean. Note that Apple computers are different and that the following doesn't apply to them.
I'll presume you're partitioning a hard drive disk (as opposed to a USB drive or something else, but the principle is the same), which i'll refer to as the HDD. There are a few ways to do it - one is if you run the Install disk of your Operating System (eg Windows XP or Windows Vista), and following through the install options it will let you wipe the partition(s) already on the HDD and create new ones to the sizes you specify. It will then write the partitions, and also format them with the appropriate file system.
Another way is to get a boot floppy disk with either Ranish Partition Manager or a DOS boot disk with Fdisk on it. Either will allow you to delete already existing partitions, and create new ones. Formatting the partitions afterwards is your job however.
Its not very simple, you do need to have some idea of what you are doing and DO NOT do it unless you are absolutely sure it is what you want.
The are also a few commercial programs that claim to re-partition a HDD without data loss. One it Partition Magic from Symantec. I've never used it but it has been around for a long time. But as danbob says, this sort of thing is not for beginners. Good Luck.
Oh, and forgot to mention, so long as it isn't your main HDD, if you're running windows you can create and delete partitions under Disk Management.