I don't use versioning for sketches yet
(I think it's time to start!), but you can work with any of the many systems available.
IMO for a single user with multiple machines, SVN is probably a good choice. You can get free SVN hosting from a bunch of places.
Basically you have to think of the repository as the parent folder for the files. When you get to a computer, you "update" the local copy from the repository, and when you leave that computer you "commit" them back to the repository (or more often, like just before making a broad sweeping change that you may want to undo later!).
There's a lot more to version control, but for what you are trying to accomplish that should get you going.
I use a free unfuddle http://unfuddle.com/
account to handle my personal web sites and projects. I know that there are others out there, but that's the only one I have experience with. I'm sure that others can recommend other products.