You'll need the Original system discs that came with your system when you bought it or a Upgrade DVD for a newer version of OSX that you bought and installed over the top of the original version. Either should boot the system and allow you to install that version of OS X.
But you will still need the original Applications DVD to reinstall the iLife Apps.
If you do not have them you can order a Replacement set from Apple for a small charge.
To boot the system from a DVD you can either hold down the C key when starting the system, that tells the system to boot from the DVD drive, or hold down the Option key when starting and that will bring up the Boot Menu screen where you select the DVD/DVD drive as the Boot source.