Understanding how to paint drywall is not difficult but to get a truly quality result it is important to start with a quality wall or ceiling. Or to quote a wise old painter who has seen it all and done it all, “If you want to make a silk purse out of a sow’s ear, you have to start with a silk sow”. It was true then and it is true now.
Start with the Right Surface
First, is this a new wall or ceiling? If so, you must make sure that all the drywall joints, corners, and nail or screw indentations have been taped, floated, and sanded properly. Look at the surface closely from all angles, first just an overview and then with a flashlight at an angle.