How can I stop my puppy to start fighting with others dogs?

Is it other dogs in the street or other dogs you own?

Dogs in the street - there isn't much you can do apart from keep a short leash, aggressive dogs are like angry people... people can go to anger therapy but dogs can't. Some dogs are just aggressive by nature.

Dogs at home - He is trying to win the spot as 'top dog' among the pack. Most dogs will establish a 'pecking order' if you have more than one dog at home and the way they show who is the most dominant is with fighting or snapping at each other until one backs down. You could try separating them and re-introduce them slowly either on leash or through a window or something like a puppy pen.

Is he really being aggressive or is he play fighting? Its really hard for some people to tell the difference--the play fights are often louder and more dramatic than the real thing. The puppy's age is also a consideration here. He may need a lot more socialization to learn how to meet other dogs properly and ask them to play politely.
If you think it is real fighting, try to find a trainer or behaviorist in your area to help work on his problem while he is still young. Neutering almost always helps also.