You just don't seem to have the type of dense muscularity that I would typically associate with bodybuilders. You have some mass, but at the same time I can't help but to think that when you cut into contest shape, you're not going to look cut, you're just going to look small.
Don't be discouraged, though. You ARE only 18. Do the show, get the experience, and learn from your other competitors. If body building is something that you want to do seriously or even just recreationally, competing will be worthwhile.