I agree with stephen84s, you need some sort of overall view of how the different parts of your system pull together.

Generally if you have an applet, it would probably have to communicate with a server that would fulfil requests such as user authentication. It would communicate with the server by means of a socket and the server would understand the communication because you would have programmed the server. I'm not sure if you already know this but I just thought I'd make a suggestion.

If what I explained is what you were uncertain about then let us know and we'll explain it further. Oh there are other ways to do this but what I described is probably the easiest and most used (client-server architecture). It's especially useful when you have multiple applets loaded from various sites. Alternatively you could create a dynamic web project... probably more flexible than an applet.