We are not told much by Shakespeare about Juliet's appearance, but may be able to surmise a few things.
One thing we know for sure is that she is very, very young. She is presently thirteen and will soon be turning fourteen. One place we are told Juliet's age is when Paris asks Lord Capulet for Juliet's hand in marriage, apparently not for the first time, and we see Capulet respond by saying, "She hath not seen the change of fourteen years; / Let two more summers wither in their pride," meaning that she is just barely fourteen, and he wants her to wait until she is sixteen to become married (I.ii.9-10). Since we know that Juliet is still very young, we know that most likely she still looks very young. She may have a fully developed figure; she may not. We know she is very beautiful, but due to her age, her face probably also still looks very childish.
We may also be able to surmise some things about her looks based on the things Romeo says about her. For example, he is frequently saying how brightly she shines. We might be able to deduce from this that she has fair coloring, including fair skin; bright, blond hair; and fair eyes, but we must also remember that this is set in Italy, and most Italians have dark coloring. If we don't want to deduce that she has fair coloring, we can at least deduce that she must have very fair skin. We see Romeo allude to Juliet's brightness the very first time he sees her. For example, he declares, "O, she doth teach the torches to burn bright!," meaning that she is brighter than even the flames on the torches emitting light and therefore teaching them how to burn (I.v.46). Hence, since Romeo frequently refers to her as bright, we can deduce that, even if she is not blond, she most likely has very fair skin.