What is a topic sentence?
Since I do not know what grade you are in, here are two sites on Topic Sentences. The first is for beginning paragraph writers and gives a very simple explanation and examples. The second is for secondary students. If you have further questions, please repost.
http://www.geocities.com/fifth_grade_tpes/longfellow.html
http://www.arts.uottawa.ca/writcent/hypergrammar/partopic.html