Start by being general. Make the sentence general but related to the paragraph. For example my topic is about how pesticides have a negative effect on the environment. So I would make my topic sentence..
Around the world there are thousands of farmers that stimulate the growth of their crops by adding harmful pesticides.
Then you would provide your three reasons and what the final sentence of your intro paragraph should tell the reader what the following paragraphs are about. =] Hope this helps.
The following paragraphs will discuss these following topics. Or the following paras will expand over these topics