You will find the answer in act four, scene one, where Juliet responds to Paris's questions. Note something important: she never says directly why she came. Paris hears what he wants to hear, and Juliet dodges with language full of subtext.
To determine why she really wants to see the Friar, examine these lines that she says in response to the Nurse's advice at the end of act three, scene five:
"I'll to the Friar to know his remedy;
If all else fail, myself have power to die."
(III, v, 241-242, Romeo and Juliet, Oxford School Shakespeare)
What two things does she say here regarding a) what she wants from the Friar and b) what she will do if he can't help her?
Back to act four, scene one: when Paris interrogates her, we learn key clues (subtext) of Juliet's real reasons for coming. Paris doesn't get it, but the Friar does! Examine these responses closely:
"What must be shall be"
(IV, i, 21, Romeo and Juliet, Oxford School Shakespeare)
(this in response to Paris's comment that they will be married next Thursday)
Are these the words of a delighted bride who is eager to purify herself in confession before her marriage? Paris thinks she's here to confess, but what is she saying about her marriage to Paris? About her other secret, which is...? (It's a twofold secret, based on her decision at the end of act three, scene five.)
- to tell that she is alredy married to romeo and take help from friar lawrence.