The first paragraph of the paper should state the topic and plan as clearly as possible. The topic and plan together are the controlling idea of your paper. If you are writing about George Washington's false teeth and their influence on the military history of the Revolutionary War, then you should say so in the first paragraph. No matter what happens, don't indent till you have stated that controlling idea. But don't ramble.
Introductory paragraphs are only introductions.
Be quick about it and get on to the good parts of your paper. Get to the point. Because we often change our minds as we write, it is a good idea to write the first paragraph after the rest of the paper is done. You may find that you have changed your conclusions or thought of new approaches. Before you start, plan to replace the introductory paragraph when you finish the body of your paper.