Upgrading to Mountain Lion
You can upgrade to Mountain Lion from Lion or directly from Snow Leopard. Mountain Lion can be downloaded from the Mac App Store for $19.99. To access the App Store you must have Snow Leopard 10.6.6 or later installed.
You can purchase Snow Leopard by contacting Customer Service: The price is $19.99 plus tax. You will receive physical media - DVD - by mail.
Third-party sources for Snow Leopard are:
Snow Leopard from Amazon.com
Snow Leopard from eBay
After you install Snow Leopard you will have to download and install the Mac OS X 10.6.8 Update Combo v1.1 to update Snow Leopard to 10.6.8 and give you access to the App Store.
Be sure your computer meets the minimum requirements:
Macs that can be upgraded to OS X Mountain Lion
1. iMac (Mid 2007 or newer)
2. MacBook (Late 2008 Aluminum, or Early 2009 or newer)
3. MacBook Pro (Mid/Late 2007 or newer)
4. MacBook Air (Late 2008 or newer)
5. Mac mini (Early 2009 or newer)
6. Mac Pro (Early 2008 or newer)
7. Xserve (Early 2009)
Assuming you never upgraded the OS since you purchased the MBP, you are still running 10.5, Leopard. You will need to upgrade to 10.6 Snow Leopard to get access to the Mac App store and from there purchase the latest version of the OS, 10.8 Mountain Lion, which will run on your MBP.
For a machine that age, might be worth looking how much you can get for it on eBay and buy yourself a new one.
Hope this helps