Check via Software Update whether further updates are required.
You should see the App Store icon in iTunes, and you now need to set up your account:
http://support.apple.com/kb/HT4479
To use iCloud you have to upgrade at least to Lion, but some functions are only available in Mountain Lion:
http://support.apple.com/kb/HT4759
You can also purchase the code to use to download Lion (Lion requires an Intel-based Mac with a Core 2 Duo, i3, i5, i7 or Xeon processor and 2GB of RAM, running the latest version of Snow Leopard, and you will have to call Apple Customer Care 1-800-692-7753 or 1-800-676-2775. to purchase it), or you can purchase Mountain Lion from the App Store - if you can run that:
http://www.apple.com/osx/specs/