Your MacBook5,1 will run the latest OS X version 10.8.5 "Mountain Lion".
If it does not have at least 4 GB memory, upgrade it. Yours can address as much as 8 GB and Mountain Lion will be comfortable with that amount.
You must purchase Snow Leopard on DVD first:
Elsewhere: call the phone number in the Apple Online Store
Apple's price is $19.99, £14.00, €18.
After you install Snow Leopard use Software Update to update it to its latest version, OS X 10.6.8.
Then, you can decide if you want to upgrade OS X to the latest version, Mountain Lion. Mountain Lion is available in the Mac App Store, which is available to you after upgrading to Snow Leopard.
To determine which Mac you have, what kind of CPU it uses, and how much memory is installed: > About this Mac
https://discussions.apple.com/docs/DOC-2455 explains more about upgrading to 10.6.
Upgrade to 10.7 or 10.8 only after making sure your applications are all compatible with that operating system, and your 10.6 system is functioning adequately and backed up.