The transition from being an individual contributor to being a supervisor is one of the toughest there is. I've written a whole e-book about the process, "So Now You're the Boss."
The transition is especially tough because both the job and the support group change at the same time. In most other promotional transitions, the job changes, but the basic support structure that a person has developed remains essentially the same. What can make things even tougher is that the new supervisor rarely understands some crucial things.
Crucial Thing 1: The transition takes time. In my research, I've found that a new supervisor who makes a successful transition goes through three stages, each of which lasts 6 - 12 months. The average time seems to be 18-24 months for the total cycle.
The first two stages I call Boss (new supervisor gives lots of orders, is surprised that giving orders alone doesn't work) and Buddy (new supervisor tries to be everyone's friend, is surprised that being everyone's friend doesn't work). Either of these two stages can come first, depending on the personality of the new supervisor.
After Boss and Buddy, those who make a successful transition move on to Balance. Here they learn how to take a bit of both and blend them into a successful personal style. My guess is that less than half of newly promoted supervisors make a successful transition, but instead get stuck in Boss or Buddy.
That's because there's hardly any training or books out there that address the transition and are helpful. In addition, many supervisory skills training programs do not address the work of supervision (talking to people about performance) and when they do, they use generalities, like "be fair" without describing what that means in practice.
Here's my quick advice to new supervisors.
You will go through a transition process much like what I've described above.
You will have two key jobs as a supervisor: accomplishing the mission through the group and caring for your people.
When you are promoted, you have less power and more influence than you had before.
What you supervise is behavior (what people say and what people do) because those are the only things you can witness and describe objectively.
The tools you have are your behavior and whatever sanctions and rewards your organization allows you to deliver.
The best thing you can do as a new supervisor is find a good supervisor who will mentor you and be a role model for you, especially during your critical transition.