Get a backup drive. This can be just about any USB external hard drive, and you can get them at most electronics stores. Try to get one that has duplex as much place as your computer, so you have room for multiple backups and so you have room for all the data you might get in the future.
When you first plug it in, Windows will in fact ask you if you want to use it as a backup. say it that you do. If you don't get this prompt, you can just go to the Start Menu, type "backup" in the search box, and hit Backup and Restore.
From there, click the "Set Up Backup" button. Pick the external drive you plugged in and hit Next. Windows' default settings are perhaps fine, so you can only hit Next and the next screen too.
On the last screen, hit "Save Settings and Run Backup". Windows will make its first backup in your drive, during which you don't want to turn off your computer. After that, it'll make regular backups in the background as you work—you don't need to deal with it again.
If you ever need to restore a file you lost, you can just go to the Start Menu, type in "backup", and go back to "Backup and Restore". You can hit the "Restore My Files" or "Restore Users Files" buttons to get those files back.