I read How to generate C # documentation in a CHM or HTML file? . but, I am using visual studio 2012, and the whole answer to this question is in visual studio 2010 and below. So how do I create it in visual studio 2012?
Sandcaple Help File Builder is my favorite.
http://shfb.codeplex.com/
You simply code as usual and generate yourself.
This is where the tutorial begins:
http://www.ewoodruff.us/shfbdocs/Index.aspx?topic=html/b772e00e-1705-4062-adb6-774826ce6700.htm
Sandcastle will certainly do the job, but I prefer DOxygen and it provides good documentation. It can also generate class diagrams for your code.