do cardio exercises! run, bicycle, treadmill! I'm totally against creatine! why? because it will increase your muscles, right, but NOT ONLY THE SKELETAL MUSCLES, HEART MUSCLE WILL INCREASE TOO!!! and believe me, that is NOT what you looking for! WHY? because your CORONARY ARTERIES (which suplies blood for the heart) will NOT increase in size proportionally with HEART MUSCLE, and that is one of the reason why you get CORONARY ARTERY DISEASE and LATER on, ACUTE MIOCARDIAL INFARCT, even STROKE, ATRIAL FIBLILATION and so on!
Normal girls don't like big muscles, that's for crazy men who doesn't have enough self confidence! That's my personal opinion!