I second what Natalia says about fitness and clean intake. If you are eager to develop self-defense skills, the first place to start is with awareness. What makes your neighbourhood feel unsafe? What can you do to avoid conflict or weirdos.
Some important points I’ve learned about self-defense, though I am a martial arts student, not specifically trained in self-defense methods, is that you need to be ready for defense with no doubts. If you feel that something is off, react right away. Move away from an uncomfortable situation. Also, be aware of how aggressive men behave. They go for weak, quiet people. If a chikan approaches you, and says something to you, respond verbally with NO! Go away! and keep your course. If you are quiet or move away from your course, it’s likely the aggressive guy will see you as a good target.
Some modern self-defense systems are taught in Japan. Osaka has Krav Maga seminars. This could build your confidence for self-defense in the short term. I have a hunch a lot of Krav Maga relies on power, which you can use in a real situation, but there will always be someone bigger, badder and more aggressive than you. If you’re up for it, Osaka has lots of martial arts. Martial arts demand fitness and have the side benefit of creating discipline and awareness of your surroundings. An effective martial arts teacher will tell you not to use muscle, but leverage and body weight and posture. Even small people become effective in self-defense. The fees for martial arts dojos vary.