First, get legal. Then know your grasses: Bermuda, centipede, fescue, St. Augustine. etc. what ever you've got to deal with. Then learn your available water, what disease attack and what will treat them and WHY they will be effective. Learn the weeds for your area and what herbicides will work on a particular grass type. Most herbicides are formulated for either Southern grasses or Northern Grasses. Know the signs of overuse, fungus, or drought/over watering as they would appear. There is a mountain of homework to do IF you are going to do this. KNOW THE COST OF EVERYTHING on a most severe case. If you have 50 lawns, you could spend upwards of 30-40K a year on JUST Fungicides. Know your overhead as well as material cost.
I'd suggest that you take a business class before starting and sinking thousands of bucks into a business and fail after a few years. Know how to deal with client objections and have your reasons ready. It's best to handle an objection BEFORE they arise. Know your client base and their available cash to hire you. Know your contract.
It's not just a game--it's a way of life.
We all are merely gleaners from the masters who were here before us.