Cannot factory reset a MacBook running OSX 10.5 using Disk Utility
There is no recovery partition on that one so no "Internet Recovery." you will need to make an installation DVD or USB key.

Log onto the app store and see if you have the option to download MacOS Lion (the latest version compatible with that Mac).

Failing that and a friend with the ability to download a compatible OS install for you, you might want to see if an Apple Genius can help you get that OS downloaded.

Failing that, it's time to hit Craigslist or eBay