Dirty talk is not for everyone.
Its hard to just get up and do and if you think about what you say then you will force your self to be thinking what the hell am i say.
my suggestion is just say the first thing that comes to your mind think about your girl naked.
Like for example: I just want to pound your pu**y so hard and for so long that you wont be able to walk for a week.
or(depending on the girl): say my name you sl**. then just bit her on the neck and shoulder area.
its all about just not thinking about it to much lol
good luck with the fu**
