Here's one option
http://www.asp.net/get-startedYou may already know this, but every browser supports HTML, CSS, and JavaScript. You'll need to learn these no matter what backend you choose. You should specifically learn HTML 5 and CSS 3. To make JavaScript easier, learn JQuery.
I stated doing web work two years ago. It was tough going at first, but I'm really enjoying it now.
>I am looking at developing an web based app that I created as a desktop based application. Please don't laugh but.... Where do I begin?
Craig Berntson
MCSD, Microsoft .Net MVP, Grape City Community Influencer