1. Programming. Don't pick a piece you know or "want" to play. When you're picking music ask yourself the question "Can my ensemble do this? Or with a little work can they do this?" The biggest mistake I made my 1st year as a HS director (my 2nd year teaching) was I picked the wrong music for my band. Pick music based on what they can play and what you can teach.
2. Record them, AND THEN LISTEN. The best money I spent is the $160 I spent on a set of Bose noise canceling headphones. They give a good representation of what is going on in the ensemble. "Podium deafness" is a real thing. You will astonished as to what you don't hear on the podium when you listen to your recording. Record and listen EVERY DAY.
3. A quality warm-up is a must. If you have the berth to write your own do it. It needs to include range building, flexibility, scales, chromatic work, articulation, and chordal balance exercises. A chorale would also be a great thing to have. Keep it simple.
4. Sight Read. Develop your system of how you'll teach the sight reading. I personally use count it, say the articulations (on da or ta), then push air (while fingering), then play. Do it DAILY. It needs to be part of your daily drill. Use the same counting system as your head director (eastman vs. traditional). Do not diverge from it.
5. PASS-OFFS. This is going to be the most difficult part. But if you neglect having them pass off the music you're shooting yourself in the foot. You don't necessarily NEED to have them pass-off every measure but at least the more difficult parts. Set the system to pass-off a portion of at least 2 of the 3 pieces and HOLD THEM TO IT. After-school detention with you if they don't, or demote them on a chair if they don't. If 2 of your 3 altos pass off, even if the non-pass-off kid is 1st chair, they lose their chair. Consequences MUST be there. But so must reward. Give them a mini candy bar when they pass off. Stickers on a pass-off chart work wonders (I know, I was against it at first too, but once I did it and I saw how crazy kids went over the star in their box, I was astonished). Do not let pass-offs go.
6. Find a mentor. If your HD is not going to mentor you you need to find someone who will. College professors, a friend, or even sign up for TMEA's mentorship program. TMEA will pay someone to mentor you at no cost to you, so long as you're a TMEA member. I HIGHLY encourage you to take advantage of that!