The first trait Juliet shows is that she is extremely sheltered. Consider where we first meet her: in her room (conversely, the first time we meet Romeo is on a street).
She is also inexperienced in romantic relationships. While her age is a good indication of that, we can also take the balcony scene as proof of this. Take this exchange:
Thou know'st the mask of night is on my face,
Else would a maiden blush bepaint my cheek
For that which thou hast heard me speak to-night
Fain would I dwell on form, fain, fain deny
What I have spoke: but farewell compliment!
Dost thou love me? I know thou wilt say 'Ay,
'And I will take thy word: yet if thou swear'st
Thou mayst prove false; at lovers' perjuries
Then say, Jove laughs. O gentle Romeo,
If thou dost love, pronounce it faithfully:
Or if thou think'st I am too quickly won,
I'll frown and be perverse an say thee nay,
So thou wilt woo...
She is so uncertain of how to act in this situation, and so she gives Romeo a few options of how she should act. Her insecurity in this shows us a lack of confidence from which he can infer her lack of experience in this sort of situation.