If you bought your webcam separately from the rest of your computer, the manufacturer of the webcam will often provide software that can be used to test the webcam and use it to make pictures and videos. This can either be found with the disc that most likely came with it or the manufacturer's website.
If it's integrated into a laptop, you'll have to download a program that utilizes it. A common program that does such is Skype, it allows you to make video calls. Your webcam is used for this functionality.
Hope this helps!