You will need to use Windows Image Acquisition (WIA) to integrate a webcam with your application. There are plenty examples of this readily available. Here is a C# Webcam User Control with source.
Here are some more articles and blog posts from people looking to solve the same problem you are:
- MSDN Coding4Fun: Look at me! Windows Image Acquisition
- CodeProject: WIA Scripting and .NET
- CodeProject: WebCam Fast Image Capture Service using WIA
- clausn.dk: Webcam control from C# and WIA
