First off you need a commercial with some tail-dragger time.

The majority of spraying operations are one-man/one-airplane shows...usually farm boys who got the flying bug, so they already understand the farming aspect.

Without an agriculture background, you would probably need to get on with a larger operation as a support person to learn the ropes. There is also at least one college program somewhere (in the midwest?) that provides flight training, ag education, and sprayer-specific training. I think it's a 4-year program though.

You might also be able to find a one-man operator that would take you on as an apprentice, but it might be hard to get stick time due to insurance unless you have a lot of hours.