it should still be an argument and it should still tell what the rest of the essay will be about. The key to a good reflection essay is to reflect critically about your own learning process. The reflection essay should be honest. You do not get points for having a better or worse experience (i.e. “this is the best project ever” will not score higher; similarly criticizing yourself or project is not the main objective.)
The reflection should also be specific to your own experience; students frequently make the mistake of being too general with statements like “I learned a lot that will help me be a future teacher.” Focus instead on the ways that you grew—and why.