First step is to call Apple at 1-800-MY-APPLE and order a copy of Snow Leopard, 10.6 for $20, install and then on-line software update to 10.6.8. You can then go to either Lion, 10.7 by purchasing a code to download that from the same phone number for $20, or if your computer is in the compatible list at http://www.apple.com/osx/specs you can just go to the Mac App Store and download Mountain Lion, 10.8.2 for $20.
Your Mac OS X version is 10.5, Leopard. First, install RAM to your computer > http://support.apple.com/kb/HT1423
http://support.apple.com/kb/sp575 If your Mac is compatible, call Apple to buy Snow Leopard. Make a backup, insert the DVD and upgrade. Finally, go to > Software Update and update to the latest version.