theres lot of ways to be a leader among your peers. you just need to think about what you have done in college. questions like this can be hard at first glance, but you really have to take time to reflect on yourself.
have u tutored? lead any study groups? been involved in any campus activities (events or clubs)? fraternity or sorority positions?
it may seem hard as first, but you need to think about everything you've done up till now. undoubtedly adcoms will ask you how you've become a leader in the past, so if you cannot come up with something now, it will only get harder down the road.
if you really dont have anything, try doing something now. craigslist some events/education gigs in your area and try to find something u can do.