Much of the previous is also applicable to our most dangerous opponent, the man. Try to avoid a fight. Laugh, make jokes, be friendly. Or be helpless, ask for advice or help. Do not look for a confrontation. There is always one bigger, faster and stronger.

Is a fight inevitable, be smarter, more flexible, faster, more efficient, act more informed. As with many techniques you cannot acquire skills by reading a book. There is need for training. There is a huge range in the field of martial arts and self defense. Boxing, jujitsu, judo, karate, aikido, taekwondo, kickboxing, wrestling, capoeira, kung fu... Krav Maga is also an efficient no- nonsense option. If you 're older and weaker aikido seems more efficient. You'll learn to use the power of your opponent. More important than what follows is that you actually do it. So go to the sports department, ask and sign up. Just the knowledge that you governed a combat sport gives you a lot of confidence, and improves your fitness and health. (In sumo wrestling, I doubt.)

When it really comes down to it, you have to turn off quickly an opponent partly, temporarily or permanently. Throw and hold sports such as wrestling, judo, I find to close and less efficient. I prefer some distance and view as long as possible. For that reason, I then chose karate.
Much depends on the club and the sensei where you turn to come. Better not choose a fighter gang, but a teacher with insight, knowledge and the right philosophy, and that can transmit this.
By training you can improve your speed, strength, agility and endurance with certain 1/3rd. Your efficiency and insight may increase by 500%.
Many techniques are based on the use of the strength of the opponent, not your own.
You learn to ward off blows and kicks, fall without hurting you, liberating yourself from a grip, holding an opponent under control or eliminate him. Hurting an aggressor (lightly) is making him more aggressive. Disable is the only option.

Some loose tips.
Put on a big mouth, bluffing and screaming can intimidate.
(Kick against some cars so the alarm goes off.)
Try to get / stay on familiar territory.
You have more power in your legs than in your arms. Kick.
Keep moving, do not stop.
You have a stable support surface if you spread your legs.
Weaknesses are: eyes, throat, (sleep), plexus (under the breastbone) and balls. Protect those of yourself, attack them to the opponent.
Make / see through feints.
Someone who stands on one leg and is kicking is less stable. Lift the kicking leg high and turn it around (it must not always, or only be karate). Or go with it for a little walk. And think of the vulnerable parts.
Elbows are sometimes forgotten, but formidable weapons. A knee kick (to groin) too.
A fist is stronger and harder than a hand.
Use what you have in reach as a weapon. Throw (several times in quick succession) with water, sand, a book... Go for the eyes.

Self-defense requires self-knowledge and self-discipline. Do not read, go to a dojo.

He had learned judo to restrain if necessary him selve with violence (Gust Gils)