how do you get promoted?

There are usually written tests. You will have to study a lot before you take the test. Some officers even take vacation time before the test, so they have time to do all the studying. For captain, it probably requires a college degree as well. The lower ranks, such as Sgt. may not. After Sgt is Lieutenant, and then Captain. Not too many people get that high. You will no longer be patrolling, as many captains sit in the office doing paperwork most of the time.