Congrats! I just finished my 12yr as a mathematics teacher. I started at the age of 21 in front of middle school kids and that was tough in itself, but three years after that I took on high school... I was always told not to crack a smile until Christmas.. but that really is not a good policy.
Think about the things that you can live with in the classroom (ex: students getting up out of their desk to sharpen a pencil during your lecture) and what you cant (ex: a student talking back or argueing with you). Then stick to it and be consistant! You can always loosen up, but after you let somethings slide, it is 10 times harder to correct an error in your discipline and then try to tighten up your discipline.
As someone else said, know your content. Be prepared for questions that could come up. A previous principal, told me to be a pro-activite teacher (anticipate questions, events, or actions that may occure in the classroom and how you will hand them) rather than a re-active teacher (react to what is said, or done in the classroom and then re-acting on instinct). If you did student teaching, pull from that experience to help guide you. Hopefully the new school you will be at will set you up with a mentor teacher, if they dont, ask for one in your department. They should be an experienced teacher not only in the subject that you are teaching but also at that school.
The main thing is note that you will not always have the answers and you are not always right. You are human and you will make mistakes... I tell my students this every year. We are in this together, and with this attitude, when you do make mistakes the students are more forgiving and realize that you are a real person. Have fun with your class and make it as enjoyable as possible. Be able to laugh at yourself and allow the kids to laugh at your mistakes.
There is always a thin line between all of this and staying professional (along with comanding authority, not demanding authority). This all takes time in developing your own style, and from one year to the next, it will change until you are comfortable with how you run your classroom. We are always our hardest critic.
Consider prayer. For all things are possible through God. He can calm your fears and guide you.