Webdesign approach issues

I have pretty general issue. I am a Software Developer who spinned into JavaScript recently and I would like to learn how to develop websites. My biggest issue is how to transform my visual ideas / sketches into HTML file.

It might be due to my lack of HTML / CSS knowledge, but I would like to kindly ask you here -- where did you begin? How do you start from a simple sketch on papper to a Web design in HTML / CSS ?

Are there any recommended courses / tutorials out there that I can learn from?