The first issue you should address is what your focus is as a writer. One type of paper would be a simple overview of his life and work. A critical paper, though, might investigate one particular comic product or how one theme plays out across multiple products. One good thematic approach is the role of women in Lee's work, as there is a significant amount of available scholarship on the topic.
The first step in finding criticism on Stan Lee is knowing which search terms to enter. Although some critical scholarship might have the name Stan Lee in titles of articles, you should also search for criticism on individual characters and titles.
Next, you should distinguish between criticism, generally published in peer-reviewed academic journals, which is what you need to cite, and fan sites, which are generally not appropriate citations.
Some of the important places to look for academic work in popular culture are the Journal of Popular Culture and First Person Scholar. Useful academic databases such as JStor and the MLA International Bibliography are available on your university library website; if you have not learned how to use these databases in your classes, a reference librarian will be able to help you.
One of the best sources for feminist approaches to games and comics is Feminist Frequency which has especially good sections on the female stereotypes (such as the "damsel in distress" or "woman as reward") found in comic and gaming universes.