Think of a main idea as an umbrella. It’s usually one sentence long. It covers everything else in your essay. The main idea itself should be a statement that has to be proved and it is also called a thesis statement. It should invite examples, definitions, and comparisons. Some ideas are broader than others; some are narrower than others.
Decide: How long does you essay have to be? Make sure your main idea is broad enough to fill out X number of words or pages in the assigned essay?
Example: All of our U.S. presidents, with a few exemptions, have had wonderful people skills. That is a main point, but it would take at least a very thick book to cover all our presidents and their people skills or lack there of.
Example: John F. Kennedy possessed a wonderful sense of humor. Better. But if my essay has to be ten pages long, I had better make sure, I have enough examples to fill that many pages.
Beware of main points that degenerate into lists:
Example: Several of our early presidents were deists.You may have to define deists, but after that a list will be all you need. Instead craft a topic sentence that explains why these presidents were deists and how it affected their decisions.
Several high school and college texts have lists of topic sentences. Some are there to provide essay ideas. Some English books have lists of sentences that are really topic sentence tests. Which of these possible topic sentences is indeed a usable topic sentence? Then you have to read the sentences and do as I did above. You have to determine why they would make poor topic sentences. I suggest you look for some of these lists, to help you identify good topic sentences and how to write them when you have an essay assignment.