Welcome to the Apple Support Communities
iCloud requires Lion or Mountain Lion. Check if your Mac is supported > http://support.apple.com/kb/sp575 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, go to > Software Update and install the latest version.
To turn on iCloud, open System Preferences > iCloud and follow the steps