The skills of combat is part of a ninja’s arsenal. To acquire defensive, offensive or both combat skills, sign up for a martial arts class. That is the best way to learn.
With an interest in ninjas, you would probably wish to train in Ninjutsu. However, if Ninjutsu classes are not offered in your area, here are other martial arts that you can train in: Jujitsu, Aikido, Aikijutsu, Krav Maga and so on.
Choose a martial art, which incorporates joint-locks and strikes for efficiency. Cross-train between two martial arts that complement each other if are able to. For example, Aikido teaches you purely defensive techniques utilising joint-locks and throws. You could cross-train with Taekwondo to learn kicks and punches.
Alternatively, you can take up video courses from a legitimate martial art organisation. After sending them a video demonstrating your understanding and moves of the martial art, you will be awarded the rank. While you may learn useful combat techniques this way, you will take much longer to master the martial art. Reason being that you have no instructor to correct you mistakes.