You can sync specific files and folders to OneDrive on your computer under "Preferences" in OneDrive's "Help & Settings" menu ...
To backup your files with OneDrive, you'll need to turn on the feature in the "Backup" tab of your OneDrive settings.
When you buy a new computer, you need to transfer important existing files from your old computer to the new device. At this point, you probably Googled something along the lines of, “How do I ...