Buckle up, kid! It's going to be a bumpy ride! Your students will test you to see what you're made of. It will feel awkward and even scary at first because you don't know them and they don't know you. But, with time, you'll learn each others personalities and you can start to enjoy your time with them.
Don't be afraid to be strict. Set the ground rules early and stick to your guns. A lack of boundaries invites a lack of respect. At the same time, be fair and flexible. If a rule or policy isn't working, discuss it with your students, change it, and move on (or don't discuss it, just change it...it's your class and you can do what you want)!
Show interest in your students lives so they know you care and so you can get to know one another. Ask them about their weekend and go to their games, plays, and art shows...they notice! Be so enthusiastic about what you do that they can't help but be engrossed in whatever it is you have to give them that day. And be prepared to laugh hysterically until your stomach hurts, even when you are trying so hard to be stone-faced, because some of those kids are so damn funny!
Try not to take it personally when kids complain, or fall asleep in class, or try to text for 45 minutes rather than listening to your amazing lesson on the Articles of Confederation...it's not you, it's them! And some kids are just pains in the a**, and will try your patience like you can't even imagine, but you'll realize they are there to teach you just how patient and clever you can be! Others will be so smart, and creative, and kind that you'll be reminded to try just a little bit harder.
Give yourself a break...in the end they won't remember how long it took you to grade their essays or that you gave them a piece of historical information that wasn't entirely factual (they probably won't even know). You're learning, too, and can only do, and know, so much! What they will remember is the time you dressed up in a powdered wig and recited the words to the Declaration of Independence or how hard you made them work to earn that 'A'.
They will roll their eyes and feign disgust when you play them the rap video you made, but in the end, they will secretly download the song to their iTunes account and will play it till they know every word! Just when you think you haven't made a difference, you'll get a letter or a card with a 'thank you' that you never even expected and it will be AMAZING and just the little bit of reassurance you need to keep doing what you do.
Your Future Self, 10 years down the road
PS- Stop buying so much crap from them...you don't need any more coupon books, cookie dough, wrapping paper, or tickets to pancake breakfasts!!