You can go to the website of Dropbox
https://www.dropbox.com or Mimedia
http://www.mimedia.com/
and install them. You get a certain amount of storage for free, or if you need more you can pay. The free is plenty for me.
Once they are installed, when you add a file to your computer it automatically backs it up. Also, you can put them on more than one computer and you can access the files from either computer.
When my computer totally died, I got a new computer, installed the software and had all my files back. I have done this a few times now. Or even if your old computer has not died, you can easily transfer all your files to a new one.
Unlike an external hard drive, you don't have to remember to back up your files, it is done automatically.