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 at least to Lion, but some functions are only available in 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.
The first step in Upgrading... is to 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... Purchase a Snow Leopard Install Disc...
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...
Next... Upgrade to ML...
With Regard to ML... Check here for compatibility of 3rd party Software you may be using...
You should also consider more RAM... Get the Maximum you can for your Mac...
It is important to get the Correct and Matching RAM
See Here > OWC RAM > http://www.macsales.com
Welcome to the Apple Support Communities
Buy Snow Leopard > http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard Then, make a Time Machine backup and/or a clone with Carbon Copy Cloner, insert the DVD and upgrade.
The disc contains 10.6.3, but you have to update to Mac OS X 10.6.8, so open > Software Update and install all the available updates. After updating, you will find the App Store on the Dock, so open it and purchase Mountain Lion. Check if your programs are compatible > http://www.roaringapps.com