Start by checking if you can run Snow Leopard:
Requirements for OS X 10.6 'Snow Leopard'
OS 10.6 Snow Leopard is once again available from the Apple Store:
and in the UK:
but nobody knows for how long it will be available.
To use iCloud you have to upgrade all the way to Mountain Lion:
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:
When you have installed it, run Software Update to download and install the latest updates for Snow Leopard.
Before you do anything, you need to make sure that your iMac model has enough RAM and supports 10.8 Mountain Lion.
Then you will need to call Apple, request a 10.6 Upgrade and update that to 10.6.8, then go to the App Store and buy 10.8.