The first step for me is a wireframe. That's just a drawing of how the main blocks will be laid out. Read Michael Parkatti's post: http://www.socialbias.com/the-case-for-building-wireframes/. Michael makes a strong argument for wireframing and reading it helped me understand my own struggle with design.
Next, build the wireframe. You drew it, now build it. Write the html and css to mimic your picture.
The next stage is filling in those boxes. You can start with the hard stuff but I prefer to crank out the easy stuff first. My goal is not to make the site beautiful (yet), just pretty enough to help me see some context.
As for the hard stuff, it depends on what you're trying to build. Digg took lots of talented people a lot of time. Make sure if you have your sights set high that there isn't a known "clone" that you can borrow as a template. Otherwise get used to learning scripting languages and frameworks. Good luck!