Start by checking if you can run Snow Leopard:
Requirements for OS X 10.6 'Snow Leopard'
http://support.apple.com/kb/SP575
OS 10.6 Snow Leopard is once again available 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.
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/
When you have installed it, run Software Update to download and install the latest updates for Snow Leopard.