Q: I know it must be simple, but how do I back-up my files to iCloud ?

Go to System Preferences > iCloud, and make sure that iCloud Drive is on. If it is, when you open a window in Finder, there should be an iCloud tab in the sidebar on the left. You can drag files to that.

Apple's iCloud isn't a Mac OS X backup service; only items you choose to put in it will be there.

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Niel has a good point, if you want to back up your system, you should purchase an external hard drive and use Time Machine, included with OS X.

iCloud Drive is not meant as a backup storage. Its purpose is to keep documents updated across all your devices and stored in one place.  If you change a document on one device it will be updated across all devices, if you store the document on iCloud Drive.

See:  iCloud Drive FAQ