First days (impressions) matter!
Show up 15 minutes early to the check in table. Chances are that you may not be needed right away, so bring a book or a magazine, a bottle of water and a snack. You never know how busy or how slow things might get, but be prepared for anything.
If the time permits, take a moment to introduce yourself to anyone and everyone, both cast and crew. It makes working with them a lot more friendly and fun, so... try to remember their names. If you feel you might forget someone's name, write it down after.
Golden rule: Act your wage!
You might have seen it in a movie or heard a story about how someone offered suggestions to someone, and these were soooo good, they got a promotion? Well, the chances are that if you're speaking without being asked a question, you will be remembered for the wrong reason. Even if you're 100% right and they're 100% wrong, do your job and offer suggestions only if asked.
So please, don't say "cut" if you make a mistake. If you screwed up bad, and want to repeat your line, repeat your line, if the director yells "cut" then and you have to reset, that's fine. It's not your job to stop filming. Even if you think your performance was sub-par, don't just stop to try again. If the director is not stopping you perhaps you're doing just what you're supposed to. If you want to try something else after, you might be given a chance to repeat, so save your comments until the end of the take.
As for other talent, if you know who they are without ever meeting them, don't talk to them unless you have to or they approach you. Otherwise, talk to them as you would any other crew member being acutely aware whether or not they are busy. Staring someone in their eyes or asking someone for an autograph while they're working or even getting ready for work is definitely inappropriate.
If you don't know what it is or what it does, don't touch it.
That applies to power outlets, so remember, charging your phone may not be a priority on the set, so if you really need to have full juice, bring a battery pack, but definitely don't unplug things or take up the last outlet and leave your charging phone unattended.
Basically, don't move anything of which you're not in charge, so practically anything. Every single object on set might be there for a reason, so even though it might seem you're doing someone a favor, just leave it. The chances are that you'll move set props and mess up a take or worse.
All sets are different.
Do keep in mind that some sets are chatty and fun with people fooling around and no one seems to mind, while others are quiet so actors could do serious work and focus on their performances. Just look at how the department heads act, if they are serious and somber, obviously don't be loud and obnoxious. It's always better to be quiet and respectful unless you have familiarized yourself with how everyone else behaves. If you're looking forward to let loose, do so if you see that it's appropriate.