Hi and congratulations! I've just completed fifteen years in the profession and although I still have plenty to learn, I've picked up a few things along the way.
1. Spend your first few days or at least a big portion of them teaching everyday procedures and rituals to your students. Don't be afraid you're being too babyish. Ex. Have a set procedures for what to do when entering/leaving your classroom and take the time to demonstrate these to your students. You can turn them into a student roleplay. Have a student demonstrate one procedure correctly and have a student demonstrate a procedure incorrectly so they see the difference. The more basic procedures you teach your classes in the beginning, the less headaches you will have later. Students at this age feel safer and more stable with set routines.
2. Have a take charge attitude, especially in the beginning. Even if you are a little nervous, do your best to "fake it until you make it." Children at this age are excellent at picking up vibes and that will affect their initial impressions of you and then their behavior.
3. I'm sure you've heard this already, but consistency really is the key. Your students need to understand that no means no, not "if I try again tomorrow, the teacher will let me" and now means now, not "in a minute".
4. Children at this age tend to, shall we say be "energetic." Whenever possible, try to get them moving around, such as in between activities. I actually play music during transitions. I have students taking turns being dj's.
5. I don't know what state you are in, but it is a good idea to get your hands on a copy of your state's eighth grade English language arts exam or any other important test your students will have to take this year. Get a head start on the types of questions your students will need to know. You can implement this into your teaching. I know in NYS where I work, they emphasize lots of vocabulary words, including being able to choose synonyms and antonyms out of four possible choices. So, from the very beginning, I have my students analyzing all kinds of everyday words and thinking about the words in creative ways to help prepare them for the kinds of critical questioning on the test.
Grassy road, I highly recommend two books that reinforce the first three points I mentioned above. One excellent book is Fred Jones' "Tools for Teaching" and the other is Harry Wong's "First Days of School." Some readers complain Wong is too condescending, but even as a veteran teacher I feel he makes excellent points.
The best of luck! You'll love it!