It sounds like you may be a classic Pear Shape, where your shoulders are narrower than your hips. This is a very common shape in western women.
Unfortunately, we are the shape our genetics dictate and there isn't much you can do to change your actual body frame. You could improve your natural shape, by eating healthily and doing a combination of cardiovascular exercise and resistance training, which will help tone up the areas you are not to happy with.
Few females are happy with their shape, you just have to be realistic, not compare yourself to other people and make the best of what you have.