how to upgrade my mac

Upgrade? From what to what?

If you have a PowerMac as indicated, 10.5.8. Is the highest possible

Start here:

http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard

but note that PowerMacs cannot be upgraded to Snow Leopard.

In addition to what's already stated, if you get an Intel Mac, see this tip on migration: https://discussions.apple.com/docs/DOC-2295