Just take on any job you can get, then work up some experience that you can stick on your resume from that and then apply for better ones. I didn't work through year 12, then got a job at Bakers', then one at Grill'd, and now I've got a much better research job because, in all that shitty dish-cleaning and floor-sweeping, I proved I would, ostensibly, do the job that was asked of me and not be a princess about it.
As for getting those less-than-wonderful jobs, just be enthusiastic and talk about how excited you are to work there, given how much you love the product. You love it. SO MUCH. You LOVE to be in a team and think it would be GREAT to be a part of a team that always looks like it is having SO MUCH FUN. Don't lay it on too thick, mind you.
Another tip - go to franchised stores, or anything where there isn't some central control filtering applications. If you can speak directly to the store manager/owner, I think you've got a much better chance at getting an interview and then getting the job.
Also, a lot of places DO hire over 18s, because there needs to be someone senior present to do money stuff or sell alcohol, if the shop does that. There were heaps of over-18s hired during my time at Grill'd.