Welcome to the Apple Support Communities
iCloud requires OS X Lion or Mountain Lion. Open > About this Mac and check Version. If it's 10.5, buy Snow Leopard > http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard Then, make a backup, insert the DVD and upgrade. Finally, open > Software Update and install the most recent version.
If it's 10.6, go to > Software Update and install the most recent version. Finally, open App Store and purchase Mountain Lion. See if your apps are supported > http://www.roaringapps.com