So, you booked a gig. Sweetness. The fun isn't over. You've actually got to do the job and at the very least not embarrass yourself.
The first thing you should do when you get on set is to ask who the 2nd Assistant Director is. This is the person who is, essentially, "in charge" of you.
Reporting to this person once you get to set is key, since you'll wan them to know where you are at all times. Nothing too extreme needs to be said once you find this person. Something simple like, "Hi, I'm AP. I'm one of your actors today."
They'll show you to your trailer, room, or to wardrobe, depending on the location & budget of the shoot.
If you are going to be anywhere other than your trailer or on set, you should let the 2nd director, or some other friend you've made with a walkie talkie know. Make-up o wardrobe will tend to be around, and if you need to run to the rest room, letting them know is alright as well.
Basically, let SOMEONE know where you are at all times. You may think that you have an hour before they shoot, but nothing's worse than finally having the sun in the right spot to get the perfect shot and they can't find the actor.
Don't be that guy. Please.