First part is registering your name or incorporating once that part is completed. If your state has a sales tax and my guess is yes you will want a sales and use tax permit which would allow you to buy stuff whole sale but also give you a tax id number to collect and send in sales tax on all your revenue.
With that done your want business insurance you may opt to lump that with the insurance on your trucks you can shop that around we have been happy with the http://www.hortica-insurance.com/
for many years now. They allowed us to set up 3 payments a year giving us 6 months off from Jan all the way to June so we don't pay that when the money isn't rolling in. What every you decide make sure you have enough to cover all your assets in case the unthinkable does happen. We have never had a real claim just 4 or 5 windows which the deductible didn't cover so they paid out then sent us the bill at-least it got us out of the bs of dealing with the customers and glass estimates.
Now the business license is not quite what you think in our state its a home improvement contractors license its 200 a year now. Due every Nov other then that unless you have a pesticide license your all set. Now go make some money.