The most important thing to put in a presentation of this sort would be a discussion of the reasons for which young people join gangs. Let us assume that you are in a school that has some problems with kids joining gangs. It can be very hard both for at-risk kids and for the other kids (and perhaps for some teachers) to understand what drives people to join gangs. If the “regular” kids and the teachers know, for example, that kids join gangs to get a feeling of belonging and being accepted, they might be able to alter their behavior to make everyone at their school feel more accepted. This might cut down on gang problems.
As for alternatives, once you have determined why kids join gangs, it is easier to identify alternatives that might give them the same “benefits” that being in a gang would. For example, if kids need to feel that they belong, a community might set up things like sports teams or drama groups that would allow the kids to feel that they can be an important part of something.
The best thing to do with such a presentation, then, would be to identify factors that drive kids towards gangs and then suggest alternatives that might provide kids with the things that they typically get out of being in a gang.