Why do you want to paint portraits? Is it to make money, or for your own enjoyment? If it's to support yourself, you'll have to be willing to do more of what the customer wants than what you want. If it's for enjoyment, you can develop your own style and can sell to those who like your style.

Get a book on the history of portraiture and learn about the phases that portrature has gone thru. Then develop your philosophy from what you learn, and shape your style around that. Some early painted portraits didn't look much like the subject, but reflected more of who that person was (socially, classwise, and work related). Other (later) artists thought that a portrait should be as near photo perfect as possible (this before photographs, of course). When you paint, do you want to put in every blemish, scar, and wrinkle? Or would you prefer to smooth out the skin and flatter the person, so that they look better? Or are you going to change your style for each person who comes thru? Know what you want to do before you get into it.

Advice for building business? Market. Advertise. And network with everybody in your area.