I'd agree the best start is probably to get an agricultural degree either from one of the Universities, or from one of the more traditional agricultrual colleges. But I'm not sure there is any absolute requirement for a degree to be an agronomist, and on the job training can be just as effective. But if you can go to University I would go - not least because it is a greta experience.
Another route in could be through field trials with an organisation like TAG, a distributor or an agchem manufacturer. I started by doing field trials for Crop Care Chemicals (now part of Agrovista) for a number of years, and it gives you a brilliant understanding of crops, at least the ones you do trials on, and what chemicals / agronomy will be required. After a year of doing trials you will almost certainly be able to recognise common weeds / diseases etc without any problems, and know their latin names too probably!
Most of the distributors are usually looking for good quality trainee agronomists too.