Start off weakly and you may as well leave because the day will be an unqualified disaster. From my experience, the first 10-15 minutes in a class are crucial. In that time the kids can size you up and see what you're all about - based on your wardrobe, stance, voice quality and confidence level. So take a few deep breaths before taking control. Get to school early enough to sign in, find the room and have it unlocked.
Schools can vary.
Pizza, trip or milk money that should be collected. Scour the Internet for resources. You'll be awed at the amount of activities, lessons and quick suggestions that are available online. Look over the lesson plan. Switch the order of some activities for variety.