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 all the way to 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.