Every research paper should have an introductory paragraph. The introductory paragraph should summarize what you will be doing in the paper. Pretend that the professor doesn't know what the assignment is. It may state what you found in your research.
Many people write their introductory paragraph last or at least modify it after they are done.
Every paragraph should have an topic sentence. There should be only one topic per paragraph. It can be helpful to go back through your paper and write the points you are making in each paragraph out in the margins. That way you can see if you are bouncing back and forth between topics within the same paragraph. Your paper should end with a concluding paragraph that summarizes the main points of your paper.