I don't know the details, but LessWrong is forked from the Reddit sourcecode - doing something similar might be a good start.

In more general terms, you need to learn to program, and write a webserver program, generally by using a Web Application Framework, and put that webserver program on a server that you either rent from any number of places or setup yourself. A reasonable way to do this (speaking from a small amount of experience making a site much less complicated than LessWrong) is to use Python with webapp2 and Google App Engine.

Beyond that, you're going to have to be more specific about your experiences and goals. LessWrong is not a simple website. Don't expect to be able to write from scratch anything of this magnitude in less than several man years (depending on what you count as "from scratch"). Building it by, say, using an existing system such as WordPress would be much, much less work.