[jarcane]

Since you're already familiar with C# you may look into ASP.NET, or even look into Websharper/F#.

Otherwise, I second the recommendation for Codecademy, for the JS/HTML/CSS or Ruby approaches.

Alternately, I quite like Racket's web tools; very easy to get started with a minimum of fuss. You'll want to know at least some HTML/CSS first, but once you get used to writing HTML with s-expressions it's hard to go back ...