Ag aviation is not an entry level business.
You can find some ag schools to attend; a number of them exist. However, once you've completed the school, you generally still have the traditional slow entry to deal with. You'll likely work on the ground for one to three years while being worked slowly into a seat. You'll occasionally be allowed to spray a little rinsate on a fallow field, and over time allowed to put some low-risk chemical on wide fields with few obstacles. Eventually you may be able to work into a seat.
The ag industry isn't doing too well right now; you may wish to reconsider your goal, in this respect. A lot of guys who might have normally done several hundred hours last year, only flew less than a hundred...didn't even make the payments on the airplane.
There are physical risks to consider. Insurance is high and very demanding, for a good reason. You need to remember that statistically, the average ag pilot has a seven year working lifespan. Some go a very long time, and some don't make it through their first season. If you are comfortable flying a performance-limited airplane at three feet amid obstacles, and turning and dealing with stalls and frequent buffets at 75' over those obstacles, as well as dealing with the chemicals and liability, then don't give up your dreams.
Presently I take a leave of absence in the summer to do fire in an ag airplane; a single engine air tanker (SEAT). Last year I got less than 25 hours from a contract that's traditionally 150 hours. Row crops and forrestry work (fertilizing, seeding, etc) are in a similiar boat. I should have had about 150 hours in December for reseeding burned areas, but so far the contracts dont' exist. What that boils down to is a loss of thirty grand in flight pay; no small amount. If that's what you use for your bread and butter, then you're hurting.
If a fully qualified ag pilot desires to earn his Level I initial attack card for fire, it's a minimum of two years (five to ten in the heavies). Again, it's not entry level, and you have to consider what you'll be doing to make a living during that time, or during the rest of the year. Many employers aren't willing to consider letting someone go for an unspecified and unpredictable time period for fire or ag (both are ag), so keeping year round employment can be difficult. Are you prepared to spend the off season driving a beer truck or working in a rubber stamp factory?
You can PM me with questions or email me. I may not be able to respond right away. I'm in the field, assigned away from home rightn ow, and have limited opportunities to post. I'll do what I can. Good luck!