The very best cleaner that I have found for plastic, or other non-metal cases, is a product called Honda Wax. It's not cheap, available at (not surprisingly) Honda motorcycle dealers. It will clean up fairly heavy stains, and the surface of the case will shine like new....
If you are asking about getting rid of unneeded files, etc, rather than a physical cleanup, then you might come back with more details about what you want to do.
Macs (and OS X in general) do not usually need a "cleanup" of files - unless your hard drive is almost full - or you want to prepare an older Mac for sale, or for someone else to use.
Let us know if that's your real issue.