How do you learn to fight?
It depends on what you want to do.
There are boxing gyms where you can learn to spar. There are many schools that teach martial arts, some of them can be very effective for self defense and others are very good for competitions.
Many health clubs teach various forms of kickboxing. Boxing is limited to the hands. Kickboxing adds some feet. Both have rules.
There are no rules in a fight. Martial arts are the way to go. The ones that don't have competitions generally contain more brutal techniques that would be against the rules of a competition.
Schools focusing on self defense generally will try to teach you morals of not fighting to be used only to defend yourself.
There are so many different styles to study but here are a few.
- Jeet Kun Do is more a way of thinking than a martial art. Bruce Lee created it and basically it takes from any art the moves that work for you. There is not a "best style."
- Wing chun is great for basics and was Bruce Lee's first art.
- Kali, escrima, arnis, are great for fighting with sticks and knives (and eventually open hands).
- Silat (if you can find a teacher) is brutal.
- Aikido uses opponents' energy against them with lots of flowing throws.
The best way to learn is to practice. Go fight and you will find out what works.