Getting promoted or hired into a management role can be really great. It's an important career milestone, and probably something you worked pretty hard for. It's hard to anticipate what it's really going to be like to be "the boss," especially since there are aspects of the job that no one really talks about. They're not necessarily negative and certainly aren't reasons to not take a management role, but it might be a lot easier to get through the first few months of transition if you knew what to expect.
Management truth #1: You are now a beginner.
The better you were at your job as an employee, the harder it might be to make the transition to being a manager. That's because being a manager requires a completely different skillset than being an individual contributor. Success is defined differently. You are still the same smart, capable person you were the day before your new role took effect. You're just applying your talents in a whole new arena. It's not like moving up from the minor leagues to the majors; it's more like moving from professional soccer to professional hockey. A few things carry over, but not a lot.
Management truth #2: Your job-related knowledge isn't as important as you might think.
Sure, if you're running a team of engineers, you won't have much credibility with them if your background is in, say, floral design. But being an expert in what you're managing can actually be a little dangerous. Being a manager is uncomfortable and vague. Your old role is familiar and safe and you know how to do it well. So you will be tempted to get involved in the things you know well and postpone or avoid altogether the tricky management stuff. Possible results? Becoming a micromanager, losing sight of the overall mission of the team, and basically abdicating your job as the team's leader. You will have to spend some time in the weeds, of course, but be careful to make sure that when you do so that it is critical (only you could do it), temporary, and kept to a minimum.
Management truth #3: Being the boss can be lonely.
Moving into a managerial role means that you can't be as open with your fellow employees, and it probably means that it's harder to get guidance from your boss. It's one thing to ask your boss how to solve a supplier issue or properly analyze the ROI on an option. It's another to ask for help with managing your employees. For one, it might feel like you are revealing that you don't know what you're doing (see #1), and for another, your boss might not be a good resource for answer to management problems. (Sometimes, they are the problem.) It can feel pretty isolating. This is where your fellow managers can be a tremendous resource. Find someone who has a similar approach as you to managing and working, and make sure you connect with them on a regular basis. They don't have to be inside your organization, but it can be really helpful if they are.
Management truth #4: You can't turn off your title.
Once you're the boss, that changes the dynamics between you and your employees, and even between you and your own boss. There are even legal implications: in the eyes of the law, you are now an "agent of the organization" because you are in a position of influence over your employees, and your actions are considered (with some exceptions) to be the actions of the organization. This means you will need to think carefully about how you interact with your employees and what kind of boundaries you maintain with them. If you are new to the organization, that is somewhat easy, but when you've been promoted from within, it gets really tricky. Don't ever say "we're all equals in this meeting," because it's not true (even if you want it to be - you have not given up your ability to influence your employees' careers by that one statement), and don't believe any team-building facilitator who tries to say that in a meeting you're in with higher-ups.
Management truth #5: You are measured on what your team accomplishes, not what you produce.
Even senior executives struggle with the idea that they may not have anything tangible to show for their efforts. Your "work" as a leader, even if you're only leading two people, is to support the team in being as successful as possible. That might mean that you spend all day talking to people and smoothing the way for your employees, without ever writing or otherwise producing anything. This can be really stressful for high achievers who are used to having and consistently exceeding ambitious, measurable goals.
Management truth #6: Your formal power is pretty limited.
On a day-to-day basis, there isn't a whole lot you can do officially to reward or punish your employees. You may have the power to hire, fire, promote, etc., but it's not like you can do that every day. In a lot of cases, you may not actually even have those powers, either. Your power will come primarily from whether your employees trust you and believe that what you're asking them to do is important. (Notice the word "asking" instead of "telling" -- it can be helpful to think of it that way even when you present a request as a statement.) Ironically, the less you use your formal powers, the more impact it will have when you do. Keep your powder dry, and try to use influence and persuasion far more than power.
Management truth #7: You have less autonomy than you did before you became a manager.
As an employee, you have your boss and maybe some of your colleagues as stakeholders, and possibly customers as well. As a manager, the list of stakeholders grows exponentially. Obviously, your employees are now an important constituency, but now you might also have other departments expecting things of you, different or higher-level customers and clients, multiple bosses (your grandboss, your great-grandboss, sometimes even higher than that), regulatory and/or legal stakeholders, such as internal counsel or external auditors, and others you may not even be aware of yet. All of these stakeholders make demands on your time and energy, and you may find that you spend your day doing an excellent impersonation of a ping-pong ball, bouncing from one meeting or phone call to the next. This can be exciting and an opportunity to do great things and build strong relationships that will help your team, but it can be somewhat disorienting for someone who is used to being able to more or less shape their workday around their own goals and needs.
Management truth #8: You are part of a system.
No leader, from front-line supervisor to Fortune 500 CEO, makes decisions in a vacuum. Everything you do with your team has potential ripple effects, within and outside of your organization. As a new manager, you may find it difficult to anticipate the effects of your decisions and changes because the parts of the system you're operating within may not be entirely clear. While some things will just have to be learned through trial and error, it can be helpful to try to map out all of the interrelationships and flows so that you have a visual representation of the system. Process maps for your team's major responsibilities can be a great start, and your relationships with your peer managers can also help you to understand some of the ripple effects.
Management truth #9: Your boss might not be able to do everything you need him/her to do.
Unfortunately, there are a lot of people in management positions who are not particularly skillful at managing employees. There are also a lot of senior executives who have forgotten what it's like to be on the front line and who like to spout off inspiring leadership quotes that don't actually help solve your problem. It may be the case that you are the most effective manager in your immediate reporting chain. Or, it might be that your boss is a great people-manager but not very politically savvy, and therefore can't help you in navigating tricky organizational dynamics. If it looks like this might be the case for you, or even if it doesn't, it's a good idea to find other executives who can guide you informally and maybe even go to bat for you formally if things come to that. You need to find a management mentor within your organization who can help you to be successful. If that person is your direct boss, great! If not, start looking around. (There are advantages to being mentored by someone other than the person deciding your raises, too, and it's always good to have connections throughout your organization.)
Management truth #10: It will get easier.
Because it has been a long time since you were a beginner at work, the challenges of managing employees may seem infinite and exhausting. If everything at work used to come really easily for you and now everything is a struggle, you may think it's a sign that you're not cut out for being a manager. That's probably not the case. If you care about your employees and care about doing your job well, you can be a great manager. It just takes time to build up those skills and get the hang of it. (If, on the other hand, you hate your employees and wish they'd just go away, then maybe management is not for you.) Make sure you're getting good support both at work and in your personal life, and cut yourself some slack. You are probably doing better than you think.