One of the tricks is to only listen to calls within your precinct or zone. If the public safe is using fixed frequencies then you only want the one for your area. If they're using a "trunking" system then you only want the "talk group" that effects you.
The codes are on-line and the more standard ones are easy to remember.
I was listening to my zone one day and heard a car dispatched to my address for some xxxx code which I didn't know. The dispatcher said "see the woman". Well, I was painting the bed room at the time, so I checked to make sure it was the right color and that I hadn't spilled any on the rug. Okay, maybe I'm not in trouble, but maybe the wife is cornered by a perp somewhere else in the house. I grabbed a gun and cleared the house not finding her. Finally I looked out and she was standing by the mail box waiting on the police. Storing the gun, I gingerly went out and ask, "honey, everything okay?"
Turns out some kids had egged our mail box and she want it reported. I told her next time to let me know before she dials 911.
I don't listen to the police anymore because I find the fire department for more interesting. The police just get dispatched and many times you don't know what happens after that. With fire department they're all the time radioing back and giving sit-reps.