From a filmmaking perspective, you have three options. Earl's option is the easy one ;-)
Beyond that, you can either go the "makeup" route, wearing a bald cap and using makeup to blend it in, or you can use CG to simulate bald-headedness. Of these two, makeup is the easy route. There are some programs out there that will allow you to "superimpose" images over parts of your face, but these are usually extremely cheesy. Otherwise, you can use a program like AfterEffects and rotoscope your hair off. Have fun editing almost every single frame!
The wisest option for video is to ignore all those folks who shout "fix it in post!" and make sure you look bald before the camera starts rolling.
For still images, the answer has already been presented. Photoshop is the way to go. If you can't afford photoshop (and really, who can spend that much for a one-time project) There's a great free photo editing program called Gimp. It works great, and again, free is a very good price.