There are 2 ways how to become a cop. The normal way or the donate way.
The normal way is similar like a long waiting list. An admin decides whether and when a
player become a cop. The admin decides his promotion (rank till police chief possible)
In the donation way you become a cop of your choice as soon as you've donated.
You can choose from time to time your cop, even SWAT.
(read the donate link for more info)