Attitude is everything. Set yourself apart from others your age. Dress well and conservatively. Groom yourself appropriately for a professional appearance (even if it is for fast food). Say "Yes, Sir / Ma'am" and "No Sir/Ma'am" to show respect and manners, until the interviewer tells you that it's okay to not say that. Answer questions honestly, and stress your strengths when asked (quick learner, good student, hard working). Have references available (at least three) of some adults in your area that can vouch for you, like a pastor, a teacher, or someone you've done a good job for in the past, and don't use family or relatives.
In my area, generally you can't work at a convenience store that sells alcohol until age 18, and I would avoid them anyway.
Grocery Store work is a good job. Fast food work is demanding, but can be fun with the right management.
Here's some other ideas:
Mowing lawns in the summer: It's hard work, but there was a kid that I went to college with that started his own landscaping business in high school. Ended up paying his own way through college, with several work crews that he managed. My stepdad supported our family well with a commercial landscaping business.
Department stores at the local mall: A little bit slower paced, but out of the summer heat. Still could have long, odd hours.
The ol' fashioned paper route: True, you'd get up very early in the morning, but you'd be done in time to go home, take a nap, and still have time to do stuff with friends for the rest of the day.
Hope this helps.