Generally, speaking, there are two types of research paper: an argumentative research paper or an analytic research paper. Each requires a slightly different focus and writing style which should be identified prior to starting a rough draft.
A single main point doesn’t have to be kept to a single paragraph, especially if you are writing a relatively long research paper. Main ideas can be spread out over as many paragraphs as you deem necessary.
The introduction is, in many respects, the conclusion written in reverse: start by generally introducing the larger topic, then orient the reader in the area you’ve focused on, and finally, supply the thesis statement. Avoid repeating exact phrases that you already used in the conclusion.
If you choose a topic, begin researching, and realize that it isn’t the right decision for you for some reason, don’t fret! Although it requires a bit more time, you have the ability to change your topic even after you begin researching others.