STUFF YOU NEED:
Your Mac comes with OS install / restore disks. Find it. This is the Install DVD.
STEPS:
Power on your computer, you'll be greeted with a short intro video.
Put the Install DVD in.
Press the two keys simultaneously: COMMAND-Q (Command key, and letter Q key) to bring up a dialog to shut down your computer. You may need to go through a screen or two until you are allow to do this. Shut down your computer.
Turn on your computer while holding the OPTION key. Keep holding option key until you see a start up disk selection, and select the circular disk.
After you select the language, go to the menu on the top. Disk Utility is under one of the menus. Select disk utility, and partition your drives.
Partition this way will erase your data. After you partition, reinstall OS X using disk.
Yes there are third party software, but this is free.
Some info on apple web site:
http://docs.info.apple.com/article.html?path=Mac/10.4/en/mh14.html
You don't really need to partition a data drive, unless you do a lot of data downloading, erasing, moving. I would keep the OSX partition as big as possible.