Your first step should be to pick up a book on ASP.NET. The book should probably be in tutorial format and include some of the topics I mention below.
This book is a good example (although it uses an older version of ASP.NET, that shouldn't be a problem).
Learn some basic programming in VB first. Then learn some ASP.NET using VB as your backend code. A little JavaScript couldn't hurt either. And you'll definitely need to know HTML. Once you get the technologies down, just take building the forum one step at a time. It will feel almost natural. First, you might add a textbox that saves a value to a database (SQL is another thing you'll need to learn). Then you might make 10 textboxes that do the same. Then you might add a login page. Then you'll learn about cookies and persisting information on the client side. You'll play with styles and learn about CSS. Just take your time... you'll get there. Just don't expect to get there all at once. :)
Submit an article or tip