well you're not going to be in a room full of accomplished adults who know more than you so don't worry... it will mostly be people in the same/similar situation to you and generally at work people are pretty friendly because it just makes things more pleasant for everyone (plus you have to be polite to people at work!)
as far as acting 'professionally' goes - you're overthinking and it's not difficult... make sure you know the dress code, I'd imagine for a brand ambassador you may have a t shirt or something to wear but generally you'll just have to not wear tracksuit or very revealing clothes, be polite and friendly to everyone, if you need to ask questions do so, be on time and work hard, as far as how to talk to managers etc who are senior to you - take their lead, some will barely speak to you and you should stay polite but keep it on work topics and serious and others will have a joke but you will soon see from watching others around you what your manager prefers