All About Forms

Before you can walk, you have to learn to crawl! First, learn some basic HTML:

Then create a forms-based interactive server on your Web:

Fill-out Forms Overview, the user interface to your server
Common Gateway Interface Overview, writing the server to interact with an HTML form

To get some idea of what you can do with an interactive program, take a look at what other people have done:

Form Examples for tutorial purposes, from NCSA
Form example with Hidden Elements, from NCSA. This allows you to send names and values back to the browser. The next time the form is submitted, these values will be transmitted back to te server. We say that this preserves state.
Play Prisoners' Dilemma, a simple game
Interactive Geometry
Best Use of Interaction, from Best of the Web '94
Orbital Velocity, a very good example taken from the Space Shuttle!
Security APL Quote Server, interactive stock quotes
Serendip's Thought of the Day

And before you're done, check out the Awesome List for other neat places!