I been marriage for 18 years and I just find out he may have another baby. I dont know what to do. ?

[Spudnic]

You're missing a key bit of info here: this child your husband has, was it conceived before or during your marriage?

If prior to your marriage, you can do nothing but embrace it. Talk to the daughter as she clearly wants to find her dad. It's not your place to intervene, but rather to be supportive.

If it was during your marriage, as in an extramarital affair, you either pack your things and leave or pack his things and make him leave.

EDIT:
I'm sorry, but if you've been married 18 years, it's reasonable to assume you're at least 36 years old now, and you should not be seeking advice on this from an internet forum. You need professional counseling, and quickly. You need to move into another room in your house, or go stay with a friend or relative for a while.
Of course he's sorry, he's going to say anything to keep you from leaving his sorry ass. But that's not your concern right now. Take some time and see a shrink before making any rash decisions.