It is still not time to compose your opening paragraph. Put your tentative thesis in writing and set it aside. Computers have made this kind of maneuver incalculably easier than it used to be. When the time comes to bring everything together in one place, you can cut and paste rather than retype or rewrite. With a tentative thesis, you can begin to put your ideas in organized form.
Outlines are particularly helpful for longer papers.
If your professor requires you to submit an outline as part of an assignment, be sure to follow instructions. Some professors may require you to write in complete sentences, others may require fragments. If the outline is for your eyes only, use any format that helps you get a handle on how your ideas are related. Whether you outline or not, you should think at this stage about the most effective way to order your ideas and deploy your evidence . Consider how and where you will address possible objections to your thesis, to your supporting arguments, to your methods for collecting data, etc.