Obviously, the most important part of the journey to becoming a French professor is becoming fluent in the language. The major of your undergraduate, graduate and possibly doctoral degree to become a French professor will be French. Therefore, take as many French classes as you are allowed to under the major requirements for your degree at your school. The more proficiency you gain the in the language, the more equipped you will be for a professorship.
Take courses in French literature taught in French to gain the facility with the language that you need to truly be a scholar of the language. Study the great novels, poems and plays of the language to gain understanding of not just the technical aspects of the language, but also how it works in a figurative and creative sense.
People with a true passion for the French language typically also have a passion for the French culture. History classes help a student to understand not only the development of the nation of France itself, but also how the French language spread and evolved in different parts of the world. For instance, French is not only the native language of France, but is also spoke as the primary language in parts of Canada, Haiti and Africa.
These might not be available during your undergraduate degree, but once you move onto graduate studies you should take seminars in teaching French. Learning the subject matter is important, but learning to teach it effectively is also critical if you want to become a professor. Always consult with your academic adviser and inform her of your career path to get advice about the best classes to take, including which educational classes might benefit you.