A business license (depending on whether or not you are required to have one in your area) - and an applicator's license if you plan on doing any fert. treatments or weed control.
At least one million in liability insurance is also going to help. . .
And you usually need to be at least 18 to sign a legal commercial contract.
MJS Lawn & Landscape
Servicing commercial and residential properties
Legal, Licensed, and Insured