At 15, fast food places are a good start.
Here in northeast FL, Micky D's starts them out at about $8 an hour. (I make that much as a CSR for a Valet Garage and I am 41!)
Dishwashing, grocery bagger and stable hand (if you live near any farms)is also good.
There isn't a whole lot a 15yo can do, really.
Most places want a valid DL or have age limitations.