The Nova meter has been designed to be quick and easy to use. As the Nova measures all parameters and weightings at the same time and has a single measurement span from 20 to 140 dB, you don't need to worry about these things before starting a measurement session.
The steps to make a single measurement are:
Many noise measurement regulations state that you should check the meter's function with a Calibrator just before making measurements. This is to ensure that the meter is working correctly and that the microphone capsule (by far the most sensitive and easily damaged part of the measurement chain) has not been damaged. During this process the meter will also make any small adjustments to take account of drift caused by temperature or pressure changes.
The Calibrator is the small black device you see at the centre of the picture above. It fits on to the microphone and emits a very accurate 1 kHz tone, usually at 94 dB. The meter detects this tone and makes any necessary adjustments. A failed calibration suggests there are problems somewhere!