Morningside, do a yahoo search for Texas Department of Agriculture. The website will have options to apply for license etc. Start there.
I can tell you from experience that it is not cheap.
You must register your business with TDA as a chemical business, obtain the applicator license, pay for the nursery/floral certificate, plus pay for and attend a continuing education class once per year.
The list above is why there are so many companies and individuals spraying illegally.
TDA Commercial Licensed Applicator
Available for sub-contract fertilization, weed control, Fire Ants and other chemicals
Sub-contract Aerifying available too