If you have used Heading1, Heading2 and Heading3 to define headings in the document it's easy.
1. Create a new section at the top of the document (with page break)
2. If you have 2007 or higher, select the References tab and Table of Contents (on the left). Pick one of the automatic options
or...
If you have 2003 or lower, I think it's in the Insert menu (otherwise, Tools).
Denis