Assuming you have 1GB or more of memory, you would buy and install OS X 10.6. It's available from the online Apple store, in the US by calling 800-MY-APPLE, or online here. After OS X 10.6 installation, run Software Update until there are no more updates available, at which point you'll be at OS X 10.6.8.
If your goal is to install iTunes 10.7, you'll be able to do that at OS X 10.6.8. If your MacBook is late 2008 aluminum or 2009 or newer, you can go to the App store to install OS X 10.8 Mountain Lion. Note it requires 2GB of memory, with 4GB recommended.