First, I'm going to state my old Chemistry teacher's advice, simply because he's said it so many times, it might be worth repeating!
Once you leave high school, you will spend a majority of the rest of your life working. Use what little time you have at the beginning of your life to not work, since it's only going to consume time from your school studies and valuable social life.
I personally didn't follow the advice. I started working at 14/15, at a coffee shop. Fun job, I did didly-squat, worked 10-11 hour days doing hardly anything at $6 an hour. Free coffee, free money, pretty much. Great job.
Then I worked at Subway for a year. Probably one of the better chains to work at, but seriously, I never want to go back into the food industry ever again.
I just got hired at a flower shop, though. (17 now.) $7.75 an hour, 8 hour work day, but pleasant enough.
If I were you, I'd wait until I was 16, just to make finding a job a little easier, since there are many places out there that won't hire unless you're 16 or above. Might be a better option to finding a possibly better job.