Start with what the Hobby actually is. Showcase the minis (a Single powerpoint slide would be best for this, as they will be to small to showcase the entire class unless you pass them around).
Next, a short History of GW, when it started and where it's been. Cover the number of revisions the games have had (8th edition Fantasy, 5th 40k ect). Talk about how Warhammer and 40k were both inspiration for the Warcraft and Starcraft series of games.
Explain how the game is played in a very basic format (i.e. You roll dice, rules can be found in the general rule book and individual codexs).
I would use powerpoint to showcase some of the more advanced minis that people have made, stuff like they have on Cool mini or not, or even here in our painting section.
Finally, end the presentation with talking about where the Hobby is going (UM Movie, Space Marine video game, 40k MMO). Discuss upcoming releases and how well the company is doing.
Hope this works for you. I would use powerpoint in addition to bringing in models.