Start by checking if you can run Snow Leopard:
Requirements for OS X 10.6 'Snow Leopard'
Whilst Apple have withdrawn Snow Leopard from their stores, you can still get it from Apple by calling 1-800-MY-APPLE (if you are in the USA) and they will supply the SL DVD for $20 for a single user, or $30 for a family pack that covers up to 5 Macs. You can also purchase the code to use to download Lion from the same number (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:
If you are outside the US call your national Apple Helpline:
If you're in the UK, use this number: 0871 508 4400
When you have installed it, run Software Update to download and install the latest updates for Snow Leopard.
To use iCloud you have to upgrade all the way to Mountain Lion:
Snow Leopard = OS X 10.6.x
It is Not available as a download... It is a Paid Upgrade.
Do this first...
Check that your Mac meets the System Requirements for Snow Leopard...
Snow Leopard Tech Specs
If so Contact Apple in your Country to Purchase a Snow Leopard Disc...
In the US...
1-800-MY-APPLE or 1-800-676-2775
After the Successful Install, run Software Update to get the latest updates for Snow Leopard.
Be sure to make a Backup of your Current System Before Upgrading...
If you're running an earlier version of Mac OS X 10.6, run the Software Update pane of System Preferences.
If you're running Mac OS X 10.5, try installing iTunes 10.6.3. If that doesn't work for what you want and you're using an Intel Mac, phone the online Apple Store and order a Mac OS X 10.6 DVD.
If you're running Mac OS X 10.4 on an Intel Mac, order the DVD as described above.
If you're running Mac OS X 10.4 or earlier on a G4 or G5 Mac, check sites such as Amazon or eBay for a retail Mac OS X 10.5 DVD.
If you're running Mac OS X 10.4 or earlier on a G3, you can't.