My interpretation of this post is that he already has 10.5.8 and has finally realised he's half-a-decade out of date and without any security protection.
Presuming the machine *can* be upgraded past 10.5.8, the correct procedure would be:
1. Make a complete backup (a clone would be preferable) using software compatible with 10.5.8 to an external hard drive.
2. Order the 10.6 retail DVD from Apple for $20.
3. Boot from the CD, erase the hard drive, and install Snow Leopard.
4. During the initial new setup process, you'll be asked if you have a backup to restore from. Connect the external drive if you haven't already and follow directions.
That should be it. Bear in mind that PPC-based apps from 10.5.8 won't even transfer over, because they won't work. So some upgrading will be in order.