@umbra21 - Actually there are computer programs which will do it for you. If you use Photoshop or its equivalent, just make sure you have an image that can be completely "bucket filled" with one touch.
That means that all the elements of the picture are touching one another and that's what you need for a successful paper silhouette.
Of course, scherenschnitte artists learn how to make something that will look really good when cut in this manner, and that's something the computer can't do for you.
Not every image is going to look good when shown without color. Although, I actually think it would be really cool to use two or more colors when doing this. Figure out the image so that each color joins together and sits on top of each other. Difficult, but I'm sure someone could do it.