Basically you need to KNOW your turf and what services can you knowledgeably do. Scout the prospective area and see what grasses you need to know about. When done, consider how to tend a healthy lawn and how to keep it healthy, a sick or declining lawn and how to recover it, know which fertilize each turf need. Centipede, St. Augustine, Bermuda all have certain and different requirement as to what, when, and how much to apply. What about weeds? Do you know them and what needs to be done to control or eliminate them? Quite a lot of homework, visits to product supply stores and people to help you. A good lawn person knows a lot of people who have similar problems and similar prescriptions.
You need to work on a small scale in order to "GET YOUR HOUSE IN ORDER" and keep records of time, applications and results. Pros do this as a requirement in record keeping. When you find a good article, print it out for future reference especially in dealing with a fungus.
Visit your John Deere Landscape place. They used to have a basic program for a lot of different grasses. Go by those and see if lawns are responding. Take a soil test for recommendation but let me warn you, a lot of them are just dead wrong. There are several great minds here that are willing to AID but not hand hold you for every stem. No cruelty meant. The learning curve will spike the first year. If you have a really beautiful lawn that exemplifies your work, put a sign in it to the effect that "THIS LAWN MAINTAINED BY......."
Finally, give 150%. Do a few free low cost things like pruning a shrub on your days off and make sure that the client know that you done this "ESPECIALLY FOR THEM" That kindness will not go unrewarded. AND LOVE YOUR WORK.
We all are merely gleaners from the masters who were here before us.