I have to agree with what Wizz said. When I first joined the game, I jumped into a police station real early and couldn't figure out how to make more credits to do anything. So after joining an alliance, I was told to drop the police station until at least at 10 fire stations. Focus on fire stations and then expand into something like EMS and maybe in police. Fire is gonna get the most credits. EMS will need some urgent cares or hospitals. But if you are in an alliance with either an alliance hospital or a player nearby has one, you'll be set for a while until you can jump into that. Police won't bring much until you get a lot of stations set up. Also be sure with police to have at least one regular police station to get some jail cells.
TLDR:
- Fire stations until at least 10 before getting into anything else will make the money
- Watch the vehicle market for required vehicles (heavy rescues are a big must once you start getting calls)
- Eventually get into doing expansions once you have a solid base of stations
- EMS stations will be good with nearby hospitals or urgent care clinics
- Police stations will be useful for expanding the additional calls you can get for your fire/EMS calls.