As ChrisW indicated, you should use Sandcastle Help File Builder (SHFB). However, even with the clean and simple GUI provided by SHFB, however, there are quite a number of issues that you may need or want to consider in order to produce a tidy, professional documentation set. My article ("Taming Sandcastle: A .NET Programmer's Guide to Documenting Your Code") documenting my experience with a variety of potential pitfalls, published on Simple-Talk.com, may save you some time, effort, and agony :-).
Once you have digested the article, and perhaps of more interest, I put a complete syntax and usage guide to SHFB and Sandcastle on this one-page wallchart, available on Simple-Talk.com. Here's a glimpse: