An outline can be thought of as an itinerary for a trip. It's an ordered list of the sites you plan to visit and what you want to do while there. If you're traveling alone and really have nothing specific you want to see or do, it's possible to discard an itinerary and go where the winds take you. But in the case of a paper, you're not traveling alone and you do have something specific you want to achieve; you have readers and you want to inform them about your topic.
An outline is also indispensable as a guide to your research. The second most common reason papers fail or get into trouble -- next to not having a clear topic -- is that the author gets swamped by the research and never reemerges from the library. An outline, even one that's quite general, will keep that from happening.
Finally, it's natural that an outline will evolve as your research progresses. It may even have to be substantially revised as new ideas occur to you. But if an outline is to be of any use at all, you should change only reluctantly and rarely. Many successful authors recommend changing the outline only after an entire draft has been completed. That's good advice. It's easier to revise and reshuffle material you've already written than it is to create it.
You'll find that faculty will be delighted to assist you with your outline. There are two reasons: it prevents you from wasting time, and it prevents us from having to fix incomprehensibly wild drafts, a massive task that usually irritates everybody involved.
There are also numerous online guides. Developing an Outline at Purdue and Organizing Your Writing are two of the best.