Where to begin on COMET

I need to set up my existing application to use comet to interact with a server (which does not yet exist). I just don't know where to begin. Does someone know of a simple application I can use to learn the techniques. I need to have the application connect to the server on start, then remain open to incoming requests from the server (via a webpage). This seems to me like it would be similar to an instant messaging program, can someone point me in the right direction?