Mac OS X Leopard is quite old, so you should upgrade to Snow Leopard or Mountain Lion. First, call Apple to buy Snow Leopard > http://support.apple.com/kb/HE57 Then, make a backup, insert the DVD and upgrade. Finally, go to Apple menu > Software Update and install the most recent version.
Open App Store and purchase Mountain Lion. Check if your programs are supported > http://www.roaringapps.com