When you search for biographical information online, it is very likely that you will find a Wikipedia article about the person you are researching. While you shouldn't cite the Wikipedia article itself, the bibliography and outside sources listed at the bottom of the page can be a great place to find additional sources.
If you are not sure if a website is suitable for a school assignment, ask a librarian for help!
If you need ideas or help choosing a person to research, there are many websites that feature well-known individuals for whom you will be more likely to find additional resources for your project.