An informative essay is one in which the writer informs his/her readers about a topic. Sometimes an informative essay is also called an expository essay. The essay does not try to persuade the reader, and it does not tell a story. The purpose of an informative essay is to solely provide the reader with information.
Most informative essays are broken down into 5 paragraphs. The first paragraph is usually an introduction paragraph, which gives the reader an overview about what the essay will be about. The body paragraphs are three separate paragraphs where the writer covers three distinct points. The conclusion paragraph is the fifth paragraph, and it sums up the essay.
The best way to choose an informative topic is to choose a topic that is interesting to you. You should also write about something that you already know about.
Keep in mind that your purpose is to inform, or tell, the reader about a specific topic. Don't try to persuade the reader to agree with you, and don't choose a topic where you are instructing the reader how to do something. That's a different type of essay. Choose a topic that you could talk about openly and freely, and write the essay as if you were telling about the topic.