Quoted from Alex_212 Hey Dan, The only way to get into their heads is by talking to a gang member, though if you do tread carefully. Alex

Take Andrew's advice for this and watch 'Ross Kemp on Gangs'.  He travels the world and manages to talk to some of the most brutal and feared gangs.

As has been pointed out, there are many types of "gangs" and the word, itself, is a stereotype. A gang is really nothing more than a group of people who are bonded together by a common interest, whether that is rejection, family, or even Star Trek. Your typical high school clique is a gang of a different color. The stereotypical street gang is generally a group of people who have been rejected by society for one reason or another, and their actions are governed by their individual needs. Sometimes that will be the need for attention, the need for money, the need for food, or the need to show rivals who is in charge (if they've reached that point under some sort of figurehead leader). the key to any story involving a gang ass a central element will be to show this group with a reason to be together and a reason to do what they do. The gang, itself, would need to be its own character with the individuals within it being individuals as well as an intricate part of the whole. This would, of course, revolve around what you're trying to say by writing a story about such a group. Are you going for sympathy for its members, empowerment for those who need a place to belong, shame to those who outcast them, or general information for those who don't understand them? Each angle will dictate how you portray this loose-knit group of similarly minded individuals, but the most important piece, again, will be how each individual finds themselves as a part of the whole and where they belong. To do it right, you can't be random and go for the sterotypes (the leader, the bodyguard, the bouncer, the geek, the new guy, the token tough girl character, the black kid who gets killed, the leader's girlfriend, the rich kid who wants to get away from daddy so he runs away and gets into the gang and hooks up with the leader's girlfriend leading to SO MUCH DRAMA, etc.)