How do I best prepare for an assignment as a First Sergeant? What is your best advice?

Having a caring attitude helps. My troops love the fact that I care for each and every one of them from the lowest private to my most senior platoon SGT. I know all of my soldiers, their families, their pay issues, etc.

Disclaimer: I had 3 1SGs in my tenure as Troop Commander. as a 1SG: you must know the regulations (to include Admin Stuff) you must be a SME at your MOS. You should know the METL of the Troop or Company. Soldier care is critical. But the biggest thing I would say is- the Commander relies on you for guidance and advice. Don't be afraid to speak your mind or disagree (in a tactful way) with the Commander, IF you disagree, dont just say- Sir this sucks. Instead say, Sir this sucks because of these issues and propose solutions. Most Commanders will listen to you if you use a tactful approach. If its wrong- say its wrong. If its a safety issue, SPEAK UP ASAP. As the 1SG, you work for the Soldiers in the Company but you must accomplish the mission.
In summary: Know the regulations, METL, speak up and look out for your soldiers (Soldier Care) and you will be fine.

I dare you to do a zonk for your first PT formation. I bet they will love you after that.

Firstly, congratulations and enjoy what's about to be the ride of a lifetime. There's a bunch of great comments here, here's my 10 cents:

1) Know your Soldiers - strengths, weaknesses, motivations, families
2) Know your mission; be cognizant of the "brandname" of your unit - You are a Subject Matter Expert of something
3) Set Big hairy audacious goals for the company and ensure your CO's goals are nested within. Constantly evolve as the company achieves. You'll have plenty of COs, but you'll be the rock
4) Make routine things routine -- Admin work shouldn't take take your time away from the soldiers. ((If you are still at Mead, get down to meet and greet the folks at MILPER on Huber Rd, they'll help with streamlining processes if you're nice :-) ))

Lastly, the NCO Creed is an absolute foundation for your job description. Have fun and lead with panache...

Congrats, you will do great. This question shows that you put a lot of thought and preperation and take this challenge as 1SG seriously to do the best. Every Company has its strengnths and weaknesses. Go into the job with a good attitude, assess your company for at least the first 30 days. Get to know the Commander, PLs, PSGs, SL's, etc. Your job is to look out for the welfare of the Company Soldiers. You know what is going on with your Soldiers/NCOs, and keep the Commander abreast if and when he needs to know. Handle business by teaching PSGs, SLs and TLs when necessary and appropriate. Most times NCOs figure it out through experince, you are there as their sounding board, a mature older adult / older Brother / Uncle / Papa Bear / Leader / Standard bearer / 1SG / Top that is always looking after the best interest and welfare of his Soldiers and NCOs. Your job is to train NCOs and set the example. Your NCOs should be providing challenging and meaningful training for your Soldiers. Your Job is to ensure this is actually happening. PT with your Soldiers/NCOs. Encourage growth and education. Please don't make your Company soldiers wait for you at the end of the day for a formation to be released. Either have the formation on time, or let them go. Nothing worse than waiting in formation for an hour, just for the 1SG to come out to say, Released. Common sense goes a long way. Company Comander plans the mission, and you make sure the NCOs and Soldiers are ready to perform. Good Luck and Enjoy, being 1SG is the best job in the Army.

Attend the 1SG course!

Don’t forget that your CSM is your adviser, not your boss or in your chain of command! Your Commander and you run the unit! Anything a CSM pushes your way is something your commander must agree with, it’s your job to show value in the CSM advice to the CO. My CO’s often didn’t want to take the CSM advice, I’ve more often than not, not taken the advice of a CSM, or outright respectfully said no,(some CSM tried to use my unit outside our mission...) but most CSM will give you good advice, so learn from them, and ask them for advice but when it comes to your unit, it’s you and the Commander!
I personally asked my first CSM to mentor me.

Praise in public and correct in private, do as much as humanly possible.

I am assuming you are a SFC promotable yes? Be yourself and apply all of the NCO traits you have learned thus far. You will learn a lot at the First Sergeant school and be sure at school to cross pollinate commo with other 1SG's as far as leadership and working with your CO (normally a Captain or Major. ) Be confident and have a positive can do attitude. Remember, your entire Company or Battery is watching you....Hoooah!

You and the Commander will be as one, and the troops need to know this.

Meet with the Commander but also meet with your Platoon leadership, get to know them well because your effectiveness relies on your leadership with them, let them know you are there for them 100% but also let them know your expectations! Be consistent and treat everyone with Fairness, Dignity and Respect. Earn respect. Be the last one to finish the day, be the first one to start it.