Relax. Your career doesn't hinge on this conference.
Maggie was right -- be professional. But don't take it so damn seriously. Try to have fun. There are SO many people at these conferences, nobody will notice if you're not Polly Perfect all the time. Don't sweat your editor appointment -- a request for a full manuscript (or lack of request) does not make or break your career. Don't be afraid to ask for an autograph, just be polite and nice when you do it. Don't be pushy, anxious, or rude -- there's plenty for everybody (plenty of food, plenty of time, plenty of autographs).
Dress comfortably and appropriately. If you're at RT, pretty much anything goes, but at other conferences, look clean, presentable, and act like a grown-up (you don't have to act like a grown-up at RT, but that's another story).
Above all ... enjoy it. It's a chance to interact with people who have the same interests as you.
Take advantage of it!