I get so nervous about what to say too -mostly because I'm not a talker -- but I am an organizer, so I write down my answers to the typical questions already mentioned by others - what brings you here to day - what do you want to change - what is your history of this problem.
I give the highlights and figure we'll get to the drill down later,
The trick is to actually bring the written answers with you so when you go blank you can look down and read from the page - or hand it over if you're not a talker.