first i always turn the heater dial to cold when starting a cold engine to allow it to heat up asap since the heater core relies on engine coolant.
if the temp gauge hasn't gone up to 1/2 way within 10 mins of idle or a mile of driving then the thermostat maybe stuck open.
when engines at or near optimum temp, turn the heater towards hot.
if the blown air is not even warm/hot, check the rod connecting the heater dial to the heater core input valve is positioned right. then check the hot/cold diverter cable is set correctly (be able to turn dial fully cold to fully hot with no resistance).
at low fan speed there should be a slight breeze. if the blown air feels very weak at hot & cold, there maybe a pollen filter after the blower that's blocked. if it feels weaker when fully hot, the heater core maybe blocked with dirt/debris.