Any device running iOS 6 requires iTunes 10.7, which requires OS X 10.6.8 or higher.
You will need to upgrade OS X on your Mac before you can update iTunes and sync your phone with it.
Contact your local Apple Store for information on upgrading. You may need to upgrade the RAM in your system to accomodate the newer version of OS X as well.
All but the very first MacBook models are capable of running Snow Leopard. It will set you back $20. Once you install Snow Leopard you can update to iTunes 10.7 and you will be all set with your iPhone.
To upgrade from OS X 10.5.8, you must first buy Snow Leopard by calling the phone number in the Apple Online Store: 800-MY-APPLE (in the US). Snow Leopard costs $20, or $30 for a five user license. Do not obtain Snow Leopard from any source other than Apple.
Make sure your system meets Snow Leopard's requirements:
To install Snow Leopard for the first time, you must have a Mac with:
- An Intel processor
- An internal or external DVD drive, or DVD or CD Sharing
- At least 1 GB of memory (RAM) (additional RAM is recommended)
- A built-in display or a display connected to an Apple-supplied video card supported by your computer
- At least 5 GB of disk space available, or 7 GB of disk space if you install the developer tools
After you install Snow Leopard 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.