How do I stop using iCloud for document storage and use it only for email? I do not want iCloud to back up my files from my computer. I only want to use iCloud for e-mail.

How do I do this?
Go to System Preferences -> iCloud (it's in the third row), and uncheck the iCloud Drive option:



It will ask you if you want to keep a copy of the files currently in iCloud, so you can immediately store them somewhere else if you want to. And you have to delete those files manually in iCloud to free up space, e.g. via https://www.icloud.com/.