How do i back up my iTunes library from my iMac to the iCloud?

Sign up with iTunes Match http://www.apple.com/itunes/itunes-match/

FAQ: Why iTunes Match [iCloud] Can Not Be Used as a Backup - https://discussions.apple.com/docs/DOC-4112

iTunes Match - http://www.apple.com/itunes/itunes-match/ - see FAQ at end too:

"What if I add new music to my library?

Do I need to tell iTunes Match to scan it?

No. iTunes Match will automatically rescan for content, so you don’t have to. But you can force a refresh by choosing Store > Update iTunes Match in iTunes."

Anything purchased from Apple theoretically also has a copy available in iCloud for redownload, but even Apple says you should not rely upon this: Apple recommends, "... back up your iTunes library. " (https://support.apple.com/HT201272). http://www.apple.com/legal/internet-services/itunes/us/terms.html - "Some iTunes Eligible Content that you previously acquired may not be available for subsequent download at any given time, and Apple shall have no liability to you in such event. As you may not be able to subsequently download certain previously-acquired iTunes Eligible Content, once you download an item of iTunes Eligible Content, it is your responsibility not to lose, destroy, or damage it, and you may want to back it up."