Scott gave good advice. Dress appropriate. No baggie jeans down to your knees or t-shirts with slogans on them. You don't have to wear a suit, but something in between.
I usually let my riders go on all my calls with me, unless its a paticularlly 'hot' call where I think you would get hurt or could unintentionally interfere. I like my riders to ask questions. Common sense says stay away from the stupid ones. "have you ever beat someone", "do you do drugs" etc. The plainly obvious insulting ones not to ask.
If you have a question about a paticular call or traffic stop, wait until its all over and you guys are back in the patrol car.
Other than that, enjoy your ride...
Be advised, I'm mean nasty and tired. I eat concertina wire and piss napalm and I could put a round through a fleas *** at 300 yards. So why don't you hump somebody else's leg mutt-face before I push yours in.