ACT NORMAL, Doreen! :)

That is the best way to act in every situation. Be calm and collected. Do not try to be someone you are not. You do not want people getting accustomed to a you that is not you. If you already have the job, take solace in knowing that they like who you are (or they wouldn't have hired you!!) so try your best to be normal and friendly. Try to make friends by talking to the people that you meet and smile a ton.

Good luck with your first day! :)