Paramedics are the pillar of society, without them, everything goes to hell. As a paramedic you have the responsibility to revive downed players, and heal injured ones. Once a player is in need of aid, your duty as a medic is to respond to the dispatch by pressing Y and rush to the spot.
Once you've arrived to the spot, get near the player and press X to start reviving them.
Once they got revived they will probably need medicine since they get up with a bit of health, so what you want to do is heal them by entering your ambulance and invite them to get in as well, once they're in the back of the ambulance, press X to heal them.
As a cop you need to fight the crime in the name of justice. Bring back the peace by taking all the bad guys to jail. Once a crime is committed nearby, you'll get a dispatch call, press Y to respond to it and rush to the place indicated by the GPS.
Once there, spot the criminal and arrest him by handcuffing him first, to do it press X when you are behind them, once done that access the arrest menu by pressing X and choose whether to take him to the PD or have some NPCs do it for you, know that taking the suspect yourself will give you more XP.
Remember that any criminal that is not a threat e.g. is not holding a gun, needs to be cuffed and arrested, while threatening ones can be killed. Red players are most wanted, this means that they have a bounty on them and can be killed by anyone, as a cop you have to get rid of them before they can cause even more trouble.