If you're running 10.6.8 as per your profile you will need to upgrade to Lion or Mountain Lion to fully utiliise iCloud.
If you decide to do that make sure your Mac meets the specs and although they will run with 2gb of RAM it won't be a good experience - 4gb or more will make a big difference.
Also, if you rely on some older programs they may not run under the new OS's and will need to be updated/replaced.
You will need to upgrade to Mac OS X 10.7 Lion or 10.8 Mountain Lion to get support for iCloud.
But iCloud is not necessary for synching your iPhone when it's directly connected to the iMac. I'd suggest you ask for assistance with that problem in the Using iPhone forum. Be sure to provide full details, including the version of iOS on your iPhone, the version of iTunes you have, and what you're trying to sync and any details about the problem.