You can write on any topic of your choosing and you can't think of anything?? My goodness. Turn off the Xbox and use your imagination. Look to your every day life for inspiration. Do you prefer to eat home cooking, microwavable stuff or carry-out food? Why? What's your favorite TV show? What TV commercials do you find totally lame and why?
You don't mention your age or location, so I don't know what kind of stereotypes you might write about. How about skateboard dudes? Why do most people think they're rowdy and rude and not-too bright? Or overweight people? How they're immediately perceived as being lazy and having no will-power.
Again, just take five minutes or so and actually let your mind wander - use your brain, or you'll lose it!