What was the process by which you got this job? Were you promoted from within the group you're now managing? Promoted from another group in the same company? Or hired from outside?
If you're an outsider, you probably have several people reporting to you who applied for your job and didn't get it. Surely a couple of them are as well suited for it as you are, it was just bad luck or office politics. So one of the things you have to do is win over some people who have a valid reason to resent you before they even meet you.
Since the demise of the position of supervisor, management now involves a lot of first-line supervision. You have to be parent, teacher, babysitter, scoutmaster, and therapist to most of your staff, to a greater or lesser extent. In IT that extent tends to be greater because the profession self-selects for people with poor people skills, people who are more comfortable in the silicon world than in the carbon world.
Don't listen to people who tell you managers just manage and your staff simply has to learn to be responsible adults. Odds are that you won't be very successful that way. You have to also supervise. So brush up on your people skills and be prepared to spend a lot of your time listening, giving advice, solving problems, breaking up disputes, and turning raw material into responsible adults who may themselves some day be qualified for promotions into management.
How to approach your first day depends so much on the specifics of the situation. As a manager your basic duty is to see what needs to be done and do it.
Hold a group meeting and get to know the people as a GROUP. You'll see the dynamics, the relationships, the hierarchy, the various strengths and weaknesses, and you'll learn a lot about the company by the way people feel about it.
Then meet with the people individually and try to get to know them outside the group. They WILL be different people than the ones you met in the group meeting. Do this ASAP, it's your highest priority, I mean like all in the first day if it's feasible. Hopefully you have enough people skills of your own, that based upon your astute observations in the morning meeting (which you guided into directions that gave you a lot of what you needed to know), you will have a good idea of how to approach each individual.
Never forget that if everything went according to plan we wouldn't need managers and you wouldn't have a job. Most of what you do will not be planned or scripted. You have to figure it out as you go along, using your instincts and your people skills.
If there's one generic thing that I try to get into the head of everyone who works for me, it's this:
There will always be problems. Things will always go wrong. The universe is like that. I will never get mad at you because something went wrong and I won't dwell on whose "fault" it is unless somebody is just not cut out for his job. What I WILL get mad about is when you see that something might go wrong and you don't TELL me about it in advance. It's my job to prevent problems, not yours.
Finally, make a clean break from your old job. You're not a technician any more. Every minute you spend doing tech work because it's so much fun is one more minute you're going to have to stay late to get your real job done.
Yes, "people skills," "people skills," "people skills." I did tend to emphasize that a little. That's because it's the most important skill in your job.