Juliet knows that her mother and the Capulet family are busy making preparations for the impending marriage to Paris. Juliet tells the Nurse to help her mother with the preparations. Juliet aldso knows that she can't carry out her plan for drinking the potion that Friar Laurence gave her if the Nurse is in the room.
In Act IV Scene 3, Juliet tells the nurse that she needs to pray and that she wants heaven to smile down on her and put her mind and soul in a stable mood to accept and deal with the forthcoming marriage to Paris. She also tells the Nurse that her mother could use help in the arrangements for the ceremony since it has happened so fast.
In Act IV, scene iii Juliet says to her Nurse:
"I pray thee, leave me to myself to-night;
For I have need of many orisons
To move the heavens to smile upon my state,
Which, well thou know'st, is cross and full of sin."
What she's actually asking of the Nurse is that she be left alone to pray. She tells the Nurse she needs to pray to get God on her side to forgive her evil and sinful ways.