Welcome to the Apple community.
It's difficult to say whether your Mac will run Lion, and hence iCloud, without knowing the specifications. The specifications required for Lion are as follows:
- An Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
- Mac OS X v10.6.6 or later to install via the Mac App Store (v10.6.8 recommended); you can install without Mac OS X v10.6 by using an OS X Lion USB Thumb Drive, available on the Apple Online Store
- 7 GB of available disk space
- 2 GB of RAM
However, it is sometimes simpler just to try to purchase Lion from the Mac App Store, the Mac App Store will let you know whether your Mac is capable of running Lion.