How is Juliet in Romeo and Juliet naïve? What are some examples?

Both Romeo and Juliet demonstrate a lack of experience and judgment in their personalities.

First of all, Romeo is very naïve to think that because he is love-struck he can simply approach Juliet at the celebration held at the Capulet home. Impulsively, he walks over to Juliet after becoming infatuated at the first sight of her. Romeo asks Juliet for a kiss, assuming that she will grant his wish:

If I profane with my unworthiest hand

This holy shrine, the gentle sin is this:

My lips, two blushing pilgrims, ready stand

To smooth that rough touch with a tender kiss. (1.5.93-96)

But Juliet will not kiss him; instead, she offers to touch his hand.

In Act II Juliet exhibits her naïveté when she speaks on her balcony before she becomes aware of Romeo's presence. She innocently believes that there is nothing in a name and Romeo would still be the same person if he were to have another name:

What's in a name? that which we call a rose
By any other word would smell as sweet;
So Romeo would, were he not Romeo call'd. (2.2.43-45)

The implication in these lines is that it is actually the last name to which Juliet alludes. In reality the name Montague does, indeed, determine who Romeo is because his hereditary connections are what involve Romeo with the feud with the Capulets. And it is this feud between the two families that is the root of all the conflicts.

Further in Act II, Scene 2, Juliet impulsively initiates her and Romeo's commitment to marry. She naïvely believes that the arrangements will all work out.

Despite knowing that Romeo is the son of her father's mortal enemy and that her family will be enraged if she becomes involved with this young man, Juliet asks Romeo when they are getting married.

If that thy bent of love be honourable,
Thy purpose marriage, send me word tomorrow, 
By one that I'll procure to come to thee (2.2.143-145).

Then, Juliet promises to meet him wherever the "rite" will be performed, and she naïvely tells Romeo "I'll lay all my fortunes at thy feet" (2.2.147) with no thought to what the future may hold. 

Romeo again exhibits naïveté in Act III when he intervenes in the confrontation between Mercutio and Tybalt. Romeo naïvely thinks that he can calm Tybalt by telling him that he has "reason...to love thee" (3.1.11), and Tybalt will soon learn the reason why he "tenders" the name Capulet "as dearly as [his] own" (3.1.41-42).

Since no one knows that he has married Juliet, Romeo's words, of course, make no sense to Tybalt, nor to Mercutio, who accuses Romeo of "vile submission" (3.1.44). Thus, because of Romeo's intervention and his naïve attempt to resolve the conflict between Tybalt and Mercutio, this scene ends tragically as Tybalt slays Mercutio by placing his sword under Romeo's arm and stabbing Mercutio.