First thing: Stand sideways in front of a full-sized mirror. What do you see? If it isn't correct, fix it, i.e., suck that stomach in, tuck that butt in, move your chest slightly forward. Ask your instructor to help you with what is correct. Get an idea what it looks like to have good posture, and then practice it. When you walk past a store window, look at yourself as you go by. If you are standing and walking properly, great, if not, fix it. It takes practice like everything else. You have to train muscles to maintain that posture and balance, so expect muscles to ache that never have before. Stomach and lower back exercises will help greatly.