Your self-image is probably one of the trickiest things you will find yourself dealing with and, unlike some issues, is unlikely to be something you deal with once in your life, or even in your day. There are myriad reasons you may have a negative self-image and I will not be able to help you with some of those. If your self-image is a recurring source of strife for you, please remember you deserve healthcare from professionals and not from a columnist with a lovely piece of vellum in Arts.
Taking care of your external self and modifying it to reflect your conceptual self are not acts of vanity. Your aesthetic wellbeing is just as important as your physical and mental wellbeing. Changing the way you look to suit yourself can be a complicated process, especially if you are unsure where to start, but I recommend small, reversible changes in the beginning. Do your hair a different way! Try on some lipstick! Wear a shirt in your favourite colour! If you don’t like the change as much as you thought you would, it is easy enough to take a shower and be back to where you started. There will be many people who tell you there are rules and restrictions to the way one should present themselves. I am here to wave around my aforementioned piece of vellum and say the rules keep changing and are made up by people who want to control others. Style comes when your ideal internal perception of yourself matches your external presentation.
Please also bear in mind you are not obliged to please anyone with the way you look as long as you are happy. Remember that you do not have to explain your aesthetic choices. Remember you are not obligated to respond to others opinions on the way you look. It doesn’t matter if they think they are complimenting you, it is only a compliment if it makes you feel better.