Hello!
To back up data, you copy all of your user files on to an external disk. You will most likely save all of you files to: 'My Documents, My Pictures, My Videos, My Music '. To back these up, go to 'My Documents' then copy everything in this file to the external hard drive. Do this for the other 3 mentioned locations - and anywhere else you save the files. Then you are all backed up! Please ask if you need anymore help
~ Angus