First, this assignment has a very specific audience of students attending a school in your community. This means you should tailor it quite specifically to the demographic and cultural nature of your region. A presentation in rural Montana, where populations are primarily white with a significant Native American minority, would be quite different from one in New Mexico, which has more people of Hispanic origin than any other ethnic group. An impoverished urban school will face different issues than a wealthy suburban one.
Your slides and associated talk should be accessible to students. Talk in concrete terms and use familiar examples instead of abstract generalizations.
Your first slide might discuss what attracts people to gangs, including lack of other things to do after school, a sense of community, a desire for respect, fear, economic necessity, and peer pressure. Each subsequent slide should address one of these issues. For example, your second slide could discuss locally available after-school activities that are good alternatives to hanging out with gangs, including sports and clubs. On economic issues, you could supply resources for finding part-time jobs. For building community, you might discuss not just local clubs and groups but online communities.
Your final slide might conclude with a list of local and web resources of potential use to students searching for alternatives to gang life or gang members trying to leave gangs.