Firstly, your scanner needs to support Motorola Type II trunking systems (essentially any trunking scanner will handle it)
What you will want to do is program talkgroup #272 (hex 011) for dispatch. It is the automated voice dispatcher, and calls will be going out just about constantly if not a one or two every minute to couple minutes.
When calls are toned out.. they will either be a high-low tone for EMS (it will sound like bee-doo-bee-doo-bee-doo) or a single long beep for fire related calls. The dispatcher will anounce what they are to respond to.
Example: [BEEEEEEEEEEEP] RESPOND ON BATTALION 8 FIREGROUND... 123 ANY STREET... REPORT OF A FIRE... . ENGINE 15, ENGINE 16, ENGINE 17, LADDER 1, LADDER 2, BATTALION 8.... [it will repeat]
If you don't care about EMS ops, don't bother with those talkgroups. When fires get toned out, units are assigned to a channel based on battalion officer in charge, so plug those all in (like above, Battalion 8 fireground for Batallion 8 calls) - you want the all talks. If you take the time to program the other tacticals, that will be helpful, but those are for sub-divisions at a scene (i.e. Interior crew will be on Battalion 8, tac 1)
Just in general, if all you care about is fire/ems, program it so you are always holding on the dispatch talkgroup, and then lock out all others, and unlock them only as units are assigned to use them.