Start by checking if you can run Snow Leopard:
Requirements for OS X 10.6 'Snow Leopard'
http://support.apple.com/kb/SP575
The OS 10.6 Snow Leopard install DVD is available for $19.99 from the Apple Store:
http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
and in the UK:
http://store.apple.com/uk/product/MC573/mac-os-x-106-snow-leopard
but nobody knows for how long it will be available.
When you have installed it, run Software Update to download and install the latest updates for Snow Leopard.
You should now see the App Store icon, 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), or you can purchase Mountain Lion from the App Store - if you can run that:
http://www.apple.com/osx/specs/
