First, you must upgrade to 10.7.2 or later:
Upgrading to Snow Leopard, Lion, and Mountain Lion
You can upgrade to Mountain Lion from Lion or directly from Snow Leopard. Mountain Lion can be downloaded from the Mac App Store for $19.99. To access the App Store you must have Snow Leopard 10.6.6 or later installed.
You can purchase Snow Leopard contact Customer Service: Contacting Apple for support and service. The price is $29.00 plus tax. You will receive physical media - DVD - by mail.
Third-party sources for Snow Leopard are:
After you install Snow Leopard you will have to download and install the Mac OS X 10.6.8 Update Combo v1.1 to update Snow Leopard to 10.6.8 and give you access to the App Store.
You can purchase Lion contact Customer Service: Contacting Apple for support and service. The cost is $19.99 (as it was before) plus tax. It's a download.
Be sure your computer meets the minimum requirements:
Macs that can be upgraded to OS X Mountain Lion
1. iMac (Mid 2007 or newer)
2. MacBook (Late 2008 Aluminum, or Early 2009 or newer)
3. MacBook Pro (Mid/Late 2007 or newer)
4. MacBook Air (Late 2008 or newer)
5. Mac mini (Early 2009 or newer)
6. Mac Pro (Early 2008 or newer)
7. Xserve (Early 2009)
Are my applications compatible?
Am I eligible for the free upgrade?
For a complete How-To introduction from Apple see Apple - Upgrade your Mac to OS X Mountain Lion.
Model Eligibility for Snow Leopard and Lion.
Snow Leopard General requirements
1. Mac computer with an Intel processor
2. 1GB of memory
3. 5GB of available disk space
4. DVD drive for installation
5. Some features require a compatible Internet service provider; fees may apply.
6. Some features require Apple’s MobileMe service; fees and terms apply.
Lion System Requirements
1. Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
2. 2GB of memory
3. OS X v10.6.6 or later (v10.6.8 recommended)
4. 7GB of available space
5. Some features require an Apple ID; terms apply.
Basic Information About Moving From MobileMe to iCloud
Thank you, I only have 1GB of memory so I will need to go with Snow Leopard
That might be worth doing for its own sake but it won't get you iCloud,, though you can set Mail up manually to access your iCloud email (you can do that at present):