You need to be part of IFATC to be ATC in expert server. Because it is expert server, not just anyone can be ATC. Head over to the ATC recruiting topic to see if you fit the requirements and how you can join.
IFATC requires a lot of study. Read the ATC manual every day and take the practice test every day. If you get 100% on the practice test, it is recommended to take the test.
As they said before, being a controller on the expert server isn’t easy and not everyone can do it. You have to go to Infinite Flight ATC and apply there, then pass a written exam and then a practice exam. After all of this, you can control Class Charlie airports. But before that check the requirements to join.
Just to add what already has been said, I highly recommend doing ATC sessions on the training server very regularly for a while and then when you are confident then try to pass the exams. There is a limited amount of attempts you can do and while the trainees will teach you, it is expected that you are already very competent with all ATC procedures.
The reason of this is easy, if IFATC Doesn’t exist, ES could convert in TS 2
Well, the hard part is studying the manual as @Aviation_BAK said. But you read it, the necessary times and you’ll be fine. Then you do the practical and, welcome to IFATC if you pass.