Not very easy to break into. In California, you actually have to apprentice with a state-licensed sprayer before you can go out on your own. Someone would have to trust you (ie know you) and be able to insure you. I suspect you would need taildragger and/aerobatic time.
Ag pilots often go to ag colleges and get ag degrees, and are often farm boys.
If you want to work for yourself, you will need an airplane, support equipment, and staff. The airplanes themselves run a million or two these days.