Max Health in Grand Theft Auto San Andreas

First You need to find an Ambulance Vehicle.

Once you got an Ambulance Vehicle you can start Paramedic Missions with number 2 on your keyboard

You need to pass 12 levels of Paramedic Missions to get Max Health.

You can do this in any city or town. Easiest Town to do this in is Angel Pine in Whetstone.
It is the smallest town and you can get Max Health very quick.

Angel Pine is Located in Whetstone. It's a small town between Mount Chiliad and the forests of Shady Creek

After passing mission The Green Sabre You can access Angel Pine

After the deadly Mulholland Intersection shoot-out with the Ballas in Los Santos, Carl Johnson is forced by C.R.A.S.H. officers Frank Tenpenny and Eddie Pulaski to perform various tasks for them, using the recent incarceration of Carl's brother Sweet as leverage. Tenpenny and Pulaski deposit Carl in the remote town of Angel Pine in Whetstone, a region of Flint County in San Andreas

Angel Pine Medical Center is a very small complex consisting of the medical center itself and a neighboring chapel. It provides parking facilities where an Ambulance spawns, along with the chapel's services to the north west where a Romero may spawn.

One you completed 12 Levels of Paramedic Missions the mission will end by its self and you get Max Health. You can go save and continue your Grand Theft Auto San Andreas Journey