Apple iMac powermac 6.1
The best answer is provided by https://discussions.apple.com/docs/DOC-6021.  This explains the maximum version of OS X which is supported by your particular system (10.5.8), as well as the maximum version of iOS which your phone can be in order to be recognised by the computer (iOS 5.x and earlier).  If you are able to meet both these conditions, you should be able to connect the phone to your computer and have it recognised (which should be the condition needed to start network sharing via USB).

It will also depend as to whether you have the optional Airport card installed in the mac, or the optional Bluetooth capability, as this will help you with your network sharing options from your iPhone.

If you've got a version of iOS on your phone that is 6.x or later, then you're probably out of luck for being able to get the phone and computer to talk to each other effectively as there's no real downgrade path available.