It's very easy if you create your sections and subsections as headers (Heading 1, Heading 2, etc.). Providing you've done this, position your cursor in your Word document where you would like your table of contents updated. Go to References -> Table of Contents (with Word 2007 and up), and insert your table of contents. Follow the dialog options to create your table of contents.
Anytime you update sections, go back to your table of contents, right click over it and select Update Field... to update with your revised sections.
Microsoft and Google has numerous excellent resources available. See https://www.google.com/webhp?sourceid=chrome-instant&ix=seb&ie=UTF-8#hl=en&sclient=psy-ab&q=word+insert+table+of+contents&oq=word+insert+table+of+contents&aq=f&aqi=g1g-c2g-m1&aql=&gs_l=serp.3..0j0i7l2j0i5.5231.7276.0.7996.8.7.0.0.0.4.158.887.2j5.7.0...0.0.sQ1qQqIZVp0&pbx=1&fp=1&biw=1440&bih=785&ix=seb&bav=on.2,or.r_gc.r_pw.r_cp.r_qf.,cf.osb&cad=b for a starting point.