Positive is dominant over negative.
You get one gene from each of your parents, so you are either --, -+, +-, or ++. Only in the case of -- will you have rh negative blood. With -+, +-, and ++, you will have rh positive blood. This makes rh positive blood the most common type of the two. This can be a problem with a baby: if mom is rh pos (+-), dad is rh neg (--), and baby is rh neg (--): attack!