Lawn Boy by Gary Paulsen

One day I was 12 years old and broke. Then Grandma gave me Grandpa's old riding lawnmower. I set out to mow some lawns. More people wanted me to mow their lawns. And more and more. . . . One client was Arnold the stockbroker, who offered to teach me about "the beauty of capitalism. Supply and Demand. Diversify labor. Distribute the wealth." "Wealth?" I said. "It's groovy, man," said Arnold. If I'd known what was coming, I might have climbed on my mower and putted all the way home to hide in my room. But the lawn business grew and grew. So did my profits, which Arnold invested in many things. And one of them was Joey Pow the prizefighter. That's when my 12th summer got really interesting.