What does Juliet's mother want to talk to her about at the party?

To answer this question, take a look at act 1, scene 3. Just before the party begins, Lady Capulet goes to see Juliet to talk to her about the issue of marriage, as she explains:

Marry, that “marry” is the very theme
I came to talk of.

What Juliet does not realize is that her parents have already consented to her marriage to Paris. Paris, a wealthy and handsome aristocratic, has asked for Juliet's hand. In response, Lady Capulet has come to persuade Juliet to accept Paris as a suitor and at the party, to get to know him a little better:

This night you shall behold him at our feast.
Read o'er the volume of young Paris' face
And find delight writ there with beauty’s pen.

Although Lady Capulet believes that Juliet is the right age of marriage and that Paris is an excellent choice of husband, Juliet is not so keen on the idea. She tells her mother that she will try to get to know Paris better but her response is less than enthusiastic. The problem for Juliet, however, is that the marriage is a foregone conclusion: it does not matter if she likes him or not, her parents have decided that she will be his wife.