Starting the engine is more than just turing the key. There is a set of procedures that must be followed for safe and proper operation of the aircraft.
PHAK Ch-07 (18) Starting System
PHAK Ch-07 (15) Ignition System
AC 91-55
AFH Ch-02 (12) Engine Starting
Cessna 172 PIM Section 4
Cessna SOPM 1-23 Priming/Starting Procedures
Cessna SOPM 1-56 Engine Start Checklist
Private Pilot ACS Page 14 Task C. Engine Starting