Great question,

You have a keen awareness that first impressions can be lasting (which is the truth), so just by the fact you’re asking means your on the right track. Every unit is made up of different people with different expectations, but I can recommend a few things that hopefully help you out.

BLUF:

Non-negotiable:
– Right place, right time, right uniform.
– Hungary to lead by example and accomplish the mission.
– Know your pre-commissioning training, but always be willing to learn from NCOs/Soldiers and unit specific SOPs
– Be genuine. You might be able to fake-out your leadership, but you’ll never be able to pull the wool over joe’s eyes. Care about your Soldier’s well-being, hold them to the standards and train hard.

Some practical stuff:
– Research the unit before arriving (history, wartime and peacetime, past CDRs and CSMs, Task organization, what the next 1 year of the unit is looking like, e.g. JRTC/NTC deployment, combat deployment, etc)
– If there is a “blue book” or standards book for the division, BDE, see if you can get a copy of that and review it. I know 10th MTN at Fort Drum had a blue book that covered all division polices specific to the division (DUI policy, cold weather uniform standards, permitted running routs, off-post PT policy, etc).
– Same thing with policy letters…some commander like to have 20+ policy letters, recommend getting your hands on them as soon as you know what unit you are assigned and before you’re informed about a policy “after the violation.” (e.g. there are normally some “black listed” bars or clubs near Army posts that Soldiers are restricted from entering (generally for their own safety), they’re normally listed in policy letters).
– Know what purpose your unit was designed to fill and ensure you know the doctrine behind that purpose.

Nice to have:
– Some commander’s appreciate a letter before arriving. Ask the S1 for a recommendation (as recommended above) I have seen this with post advanced course (MCCC) CPTs who know what BDE they are going to and write a letter to their future BDE commander.
– Try to get a training area map or at least be familiar with the training areas, if you don’t get it before you arrive, get one once you hit ground and acquaint yourself with the training areas. Get to know the training areas (and training aids/TSC/simulation centers) your post provides. It’s very helpful to know what ranges have what capabilities. As a leader you help provide the best resources to train your soldiers, it’s good to know

Hope that helps.