running OS X 10.5.8 on macbook, want to install iCloud, how can I upgrade

If you don't have at least a Core2Duo MacBook, you won't be able to use iCloud in Mac OS X. A CoreDuo or CoreSolo MacBook is not compatible with 10.7. It is compatible though with Windows 7, which is compatible with iCloud. Some iCloud services work with pre-10.7.2 systems, but not all.

Rosetta is not compatible with 10.7. So you won't be able to run applications made for pre-2006 Macs.

That said, read these tips for upgrading to 10.7:

https://discussions.apple.com/docs/DOC-2465

10.8:

https://discussions.apple.com/docs/DOC-3761

And getting iCloud

https://discussions.apple.com/docs/DOC-2551

Some links on the iCloud tip that may be useful no longer work, so asking on the iCloud forum is probably best:

https://discussions.apple.com/community/icloud/icloud_on_my_mac