BleepingComputer.com: low disk space on D and F

For a couple weeks, I kept getting an error message saying that I was critically short of any more space on disks D and F. I tried everything but just gave up and called Dell. The man took over my compt and eliminated the problem for $49.99.
Then he said to prevent this in the future that I needed an external hard disk so I could back up files, etc. I did buy it for $79.00 because I really don't want this happening again.
How difficult is it to add an external hard drive? It's an Iomega with 350gb, I think. Or maybe it's 500 gb.
Might someone please be so kind as to respond to this?
Thank you leeflea.

While computer off....
Connect power cable to any power source and USB cable to any USB port on your computer.
Start the computer and you should see the drive in My Computer/Windows Explorer.
Click on it.
If no message, it's ready to go.
If it's not formatted, it'll ask you to format. All you need to do is to click OK.