If I talked with a girl about the privates areas, is it a sin?
I have to make some assumption about why you wish to talk about genitals with a girl. I assume that this is not a nurse or a doctor and that you are wanting to do so because it gives you a thrill.
"But fornication and all uncleanness or covetousness, let it not even be named among you, as is fitting for saints; neither filthiness, nor foolish talking, nor coarse jesting, which are not fitting, but rather giving of thanks. For this you know, that no fornicator, unclean person, nor covetous man, who is an idolater, has any inheritance in the kingdom of Christ and God. Let no one deceive you with empty words, for because of these things the wrath of God comes upon the sons of disobedience. Therefore do not be partakers with them" (Ephesians 5:3-7).
Texting and talking to your girlfriend about sexual things is going to focus your thoughts on sex. But this is something you should not be engaging in, since you are not married. Instead, you should focus on the non-sexual things that you like and enjoy with your girlfriend. Make it a rule with yourself and your girlfriend that sexual topics are off-limits until after marriage.