In Act I, Scene 2, lines 13-33.  Capulet tells Paris to woo Juliet and If he wins her heart, then he will give his consent.  He asks him to join them at the feast that evening and also be aware of the other girls that will be there.  He wants Paris to be sure and does not want to see his daughter hurt because she is not yet 14 years old and this is a big step in her life.

In Act I, scene ii, Lord Capulet tells Paris that Juliet is only fourteen and he would prefer to wait two more summers, or at least until she is sixteen, to start thinking she is ready to consider marriage. He believes that girls who marry so young grow up too soon. However, he tells Paris to go ahead and charm her and make her love him. His permission is only part of Juliet's decision, and if she agrees to marry him, he will give them his blessing.
Act I, Scene ii, 7-11
" My child is yet a stranger in the world.
She hath not seen the change of fourteen years.
Let two more summers wither in their pride
Ere we may think her ripe to be a bride."
Act I, Scene ii, 13-15
And too soon marred are those so early made.
Earth hath swallowed all my hopes but she.
She's the hopeful lady of my earth.
But woo her, gentle Paris, get her heart.
My will to her consent is but a part.
An she agreed within her scope of choice,
Lies my consent and fair according voice.

Her father tells Paris several things. First of all, he says that she is too young for marriage. She is only thirteen. He thinks she should wait until two more summers have passed. That would mean she would have turned 16 then (lammas tide is August 1st). Capulet loves his daughter. She is his only child (the earth hath swallowed up all of my hopes but she) and he says his consent is only a small part of the bargain. He will only agree to a marriage if she consents first.

This is not the first time he's said no to Paris. He says he's simply repeating what he's already told him. He also tries somewhat to dissuade Paris by suggesting that he come to the party and look at other women, seeing if he won't find another equally attractive. Capulet (at this point) is not all that interested in marrying off his daughter, even to someone as well-established as Paris.