First, post to the correct board (Software in this case, not howto since this is not a tutorial but a question).
1. Save everything you need to keep. Double check that you have everything on the DVDs, other Macs or where ever you want.
2. Insert the system discs to the computer, boot to the install CD/DVD; at boot hold down C key. Select the source, and ERASE AND INSTALL of the install options. Or even better, from the install menus there should be a link to Disk Utility (it was called something a bit different in 10.2.8 but don't have the discs on the reach to check which) if you can find it from the menus, and select "erase disk" from its menus, THEN do Erase and Install. This will clear everything you had on that disc.