Love, eat healthy foods. Be balanced. Exercise. Take the best care of your hair and skin. Don't be afraid to dress well and use cosmetics. I remember a comedian once said, and he was black, "Don't try telling a 350 lb black woman she aint sexy." And he spoke of how black women tend to care more about dressing well and looking their best no matter what size.
Beauty doesn't come in a size. A woman can be beautiful, slender and boring. If she's got a natural sex appeal and charisma, she's going to have it no matter what she weighs or how old she gets. Confidence is key.
If anyone pressures you to look a certain way, or to be a certain size, remember that it is his/her issue, not yours. Shallow people are unpleasant. I'll take fat and jolly any day over a twit, fat or thin.