There are many small steps to this one action. It involves maintaining your personal grooming by bathing regularly, brushing and flossing your teeth, and eating tasty and healthy food. It also means making time for yourself, even when you are very busy and when others monopolize much of your time.
It may not seem this way, but when you take care of your basic needs, you are telling yourself you deserve the time and attention it takes to practice self care.
Learn to believe in yourself. You don't have to look like Brad Pitt or Jennifer Lopez to start building your self confidence. All you need is to take care of yourself and always tell yourself that you're worth caring about. This way you will start developing some level of good confidence.