These instructions are for Mac. Any Mac. If you are on a PC, get a Mac. 
1. Download Dropbox
You need an invite from a cool techie friend.
2. Install it
(Yes, drag and drop it to your Applications folder)
3. Fire up a Terminal.app
(You can find this in Applications/Utilities/Terminal.app with the Finder. Double-click on it)
4. Type:
cd ~/Dropbox
You are changing directory to the default Dropbox folder.
5. Type:
ln -s ~/Documents Documents
Basically, I created a symbolic link in my Dropbox folder to my Documents folder, which did the trick. This creates a backup of your Documents folder, by creating a symbolic link in your Dropbox folder.
And you are done! Dropbox will automatically upload your files to a Dropbox/Documents folder.
