First of all, energy. Lots of it.
Second, save every piece of paper anyone gives you that might serve as a lesson someday. It's hard to know how to file them, but if you substitute or observe someone, ask for copies. Things can be adapted across the curriculum.
Ask questions and write down what you learn.
When you start to take additional classes, try taking writing strategy classes, even if you're not an English teacher. Writing is important in every facet of education, and in these classes, with teachers that don't teach English, I have gotten some super ideas to adapt.
Save every piece of documentation given to you. Every note from parent, student, guidance counselor, principal. This may sound paranoid, but you NEVER know when someone is going to try to pull a fast one, and leave you holding the bag. This is especially important with special-needs students with what we call IEPs, which are like contracts between the school and the student and family regarding individualized instruction. Whatever is in those guidelines, follow them. Maintain close communication with the Special Ed. liaison for that student, and with parents. Print out a copy of everything you send so the computer doesn't "lose" it, and keep it somewhere that you can find it.
When working on the computer, stop every 5-10 minutes and save. And save. And save. And print out a hard copy that you keep for yourself, in case a computer or classroom file disappears.
Bring patience: for your students and yourself. The kids will test you. Being their buddy rather than their teacher will probably backfire, but as you get to know the kids, you can hold them accountable while still letting them know you care.
Don't expect that you will be perfect. You won't be, but your passion will count for a lot. Be kind to yourself.
Don't believe everything you hear. Go to the source, ask polite, leading questions, and try not to lose your temper. You NEVER know when you'll need something from someone: input on a student, coverage for a class. And kids? They love to tell you TALL tales about everything: not to be mean, but because they think it's funny.
Be your own critic. If you have done your very best, that's the most important thing—learn from your mistakes. After all, if you make a "mistake," once you learn from it, it becomes a life lesson. Don't let others beat you up. They won't need to if you're checking your progress and work ethic.
Apologize if necessary, even if it's to your class. Let them see that you are human. Don't argue with kids in front of the class. Don't meet with a student alone in a class, but in Guidance, out in the hall, etc.
Don't cut off more than you can chew: if you give an assignment, remember who has to grade it.
If you have your own supplies, lock them up or they will walk...could be a teacher or a student. (No purses, wallets or laptops unattended in school.)
Let your passion inspire your kids. Even those who don't seem like they care will take something positive from your class. And don't feel like you're to blame if you've tried really hard, over and over, and a student just cannot or will not "jump in." These guys bring a lot of heart ache with them, and sometimes school just doesn't cut it for them.
Whew. That's how I see it. Hope it helps. Carpe diem!