how to open web cam

I think that solution can be helpful to you. There is a sample you can download and try it, and get to know more information how to control your webcam. If you click WPF tab you can also find solution too:

http://www.camera-sdk.com/p_35-how-to-connect-to-an-usb-camera-and-display-the-picture-onvif.html

The solution will depends on what kind of driver the web camera has. Most of them come with DirectShow and TWAIN drivers.

If the camera has DirectShow driver, as far as I know the DirectShow SDK doesn't have .NET interfaces.
You can try to use some wrapper classes like the one described here:
http://www.codeproject.com/Articles/2615/DirectShow-NET

Or you can use a professional toolkit such as leadtools that has a .NET interfaces for DirectShow. It also has .NET Twain functions if your camera has a Twain driver instead of a DirectShow driver. For details see this:
http://support.leadtools.com/CS/forums/10345/ShowPost.aspx