Once you install it, it's just a folder on your desktop.
Put all your photos/files/things-you-dare-not-lose in there. The folder saves to dropbox servers (just think of it as a copy of your hard drive in someone else's house) meaning your files are safe.
It's totally automatic, so there's...
As you're trying to catch up with tech, here's some more advice.. No one "goes out to buy" anymore! Look on Amazon, they're much cheaper.
Also, try Dropbox. But one step at a time ;-)