OK, so the user puts this URL into his browser, a request is made to the server, and the page comes back to the user. There are two general approaches you can use to filling in the form details. 1. You can make it happen on the server. 2. You can make it happen on the client. If you want to make it happen on the server then you're going to need to use a server-side technology like ASP.NET, PHP, JSP, etc. If you want to make it happen on the client then you'll need a client-side technology that will almost certainly be javascript.
There's a whole lot more to say about this, including warnings about security holes like cross-site scripting, but I'll leave those for now.