Do you know what you're nervous about? Are you worried something is wrong, or worried baby won't cooperate and you won't find out gender, or are you just nervous-excited?
Either way, calm down, nothing you can do except wait. Go treat yourself to whatever breakfast you want, ANYTHING!! Go browse baby clothes and maybe even buy one outfit in each gender to take with you to the appt.
And of course, this goes without saying, tell us how it goes as soon as you can!!!!!