Where does it tell you how to get customers?
You may be a programmer now, but once you start your own business, you are now a businessperson first, and a programmer second. This is a critical change of state that you must make. Like almost everything else, you shouldn't count on anyone telling you anything. You are no longer someone's employee. You must initiate the finding of all "how to's".
You are correct in believing this is the biggest obstacle. Without customers, you are dead. With them, you can solve almost any problem you have.
As far as getting them, you can (and should) try just about anything. Some of my experiences: