Just click on Help in the OneCare GUI, when you get to the Instant Help (Windows Live Help) screen, enter "Back up your files" without the quotes in the search box and click the Submit search button. That section will explain exactly what you'd get told here.
http://onecare.live.com/standard/en-us/3/helpsupport/default.htm#
You'll also find that if you simply click 'Backup your files" in the OneCare GUI, a backup wizard will open and guide you through everything you need to do. Since most people only do this once, we'd have to go back through it to tell you anyway, so just try it yourself.
It's more important that you learn how to use the built-in help, since there often isn't anyone around to answer your questions here for up to a day or more. If what you read there doesn't seem to work or you're confused on a specific point, then these forums are useful.
OneCareBear