To makeuse of iCloud you will have to upgrade your Mac to Lion or Mountain Lion, provided it meets the requirements.
The requirements for Lion are:
- Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
- 2GB of memory
- OS X v10.6.6 or later (v10.6.8 recommended)
- 7GB of available space
To purchase it you will have to ring Apple at the number given at the bottom left of this page.
The requirements for Mountain Lion are listed here:
It is available from the Mac App Store (in Applications).
You should be aware that PPC programs (such as AppleWorks) will not run on Lion or above; and some other applications may not be compatible - there is a useful compatibility checklist at http://roaringapps.com/apps:table