I started out doing lawns the same way you did. It started out with somebody knowing someone that needed their grass cut, and it's went from there. I've also learned that word of mouth is the best way to advertise. Also, just talking to people about what you do and things like that.
I've also learned that you have to be willing to stand and talk with the customer for a couple of minutes once in a while. You can't just cut the grass and leave.
Anyway, this is just my .02 and the major thing of all that I have learned is to buy the best equipment you can afford.
Good luck and enjoy making your money.
By the way, what are you going to haul your equipment around in?