The best way to learn any martial arts or to learn how to defend yourself is to get in person training. However this is not always possible. There may not be adequate training facilities in your area, or maybe the training schedules don’t fit your work or personal schedule.Or maybe you are already practicing a martial art and you want to expand your knowledge of other fighting / combat arts.
In all these cases an online martial arts / self defense course can be a great solution.
Udemy offers many different Martial Arts and Self Defense Courses. You can check them out here.