As a psych nurse for 25 years, I can say the most important thing is SAFETY, SAFETY, SAFETY!!!---yours, your co-workers', and the patients'.
The first thing I learned in psych was "never let the patient get between you and the door". Always be aware of your escape route and where patients are at all times relative to you. Don't leave dangerous objects sitting about, and always lock or close doors that are meant to be locked or closed.
Don't cut corners on suicide checks, body searches, or any other such security procedures, because the universe will find a way to make you regret you did.
Do not engage in or support others in "staff-splitting"--apply rules and privileges equally to everyone, as far as their treatment allows. If you enter a situation where another staff person, even one "below" you in the hierarchy, has made a decision concerning a patient issue do not countermand them unless their decision is grossly inappropriate.
Otherwise enjoy yourself, be yourself, and take the opportunity to experience as much as you can. If you plan to work there a lot, you might ask the staff who the "regular" patients are, and get to know them.
PS: Remember--in most states, unless a patient has a court order to force meds or other treatment forms, they have all the same rights anyone else has: to refuse meds, and to leave any time they want, unless they are involuntarily committed. Your mileage may very depending on what state you're in.