Just visit the Dropbox site, register for a FREE account, and download and install the software. When alls said and done you have a new "Dropbox" folder on your computer and anything you put in it is automatically copied to all of the other computers that you have installed DropBox on. And anytime you change a file or delete something or move something the change is duplicated everywhere else. Simple! It even works between platforms - I'm running it on Mac OS X and Ubuntu right now without any issues.
Now for the downside. Although they are encrypting everything and storing it on Amazon's S3 service I wouldn't go putting sensitive material in your Dropbox if I were you. Dropbox is great for your Mom's "secret" pot roast recipe but not the Colonel's 11 herbs and spices. It's highly unlikely that someone would care about your stuff and even if they did it would be tough to get to it, but why take the chance? Actually this advice doesn't just apply to Dropbox - if you don't want people to see your sensitive stuff, dont put it on the 'net.