You'll want to start up from a CD or external hard drive, you can't be booted from the destination drive. Of course, partitioning will reformat the drive in question. When you're booted from another disk, open Disk Utility (in the Utilities folder), select the drive, and the rest is pretty much self-explanatory.
The point I would make, however, is that partitioning is usually a bad idea, one that's regretted later. It carries almost no benefits. Give yourself 8-10 gigs for the boot volume (making an Applications partition is a very, very bad idea, Software Update will get screwed up, apps won't launch right, and installers will usually put apps on the boot volume... so your boot volume must also be your App volume). So if you make the boot volume to small, you'll be limiting the amount of apps you can have.
Partitions may be helpful when you're working with extremely large files that you don't want fragmented together with your boot files. Or when you need a scratch disk, files that are created/erased very often.
So unless you have a real, specific need for partitioning, avoid future headaches by sticking to a single disk.
paul