Understand that all things in the signal chain are relative to the sum of the parts.
If the singer is too loud you have to adjust in one direction. Too soft and you make adjustments accordingly.
So let's try a simple experiment.
Have the singer stand in front of a microphone and sing a bit. I generally reach for a Neuman U87. It's a great mic and versatile enough to accommodate most vocalists. Of course, there are many different option so use what you have.
Begin by altering their proximity to the microphone. If they are singing loudly you may want to move them away from the mic. Mics are all different and have different "thresholds" for how they handle oncoming sound. If the vocalist is too close to the mic you can experience what it referred to as "proximity effect." The resulting sound is very distorted and sounds like the mic has collapsed.
Adjust the mic preamp to suit the vocalist and the mic. The condenser mics require less gain from the preamp. They usually have their own power supply, plus the added 48V of phantom power.
Let's say the vocalist is singing at a moderate level. You'll need to adjust the preamp's input gain to accommodate the mic. Too much and it can be prone to distort. Too little and the tone will change. The low level recording can be particularly problematic, as it can result in poor signal to noise ratio.
Now adjust the output so that the levels going into your DAW or recorder are at optimum level.
In many situations you'll want to incorporate a compressor into the chain. This will usually come immediately after the preamp.
The same care needs to be applied to this unit as well. It will need enough output from the preamp to work properly. Check the input meters on the compressor. Now either back off or increase the threshold of the compressor until you hear it compressing the vocal. Now that you have compressed the signal, by say, 3db you'll need to make it up with the output gain.
The output gain being sent from the compressor to the DAW should be at a similar level to what it was before including the compressor.
I typically have my setup calibrated at -18dbFS. This allows my digital levels to be the same as 0dbV on analog equipment.
"FS" stands for full scale. 0 dBFS is the level of a signal that is at the maximum level your system can cope with without clipping the tips of the waveform.
So -18 dBFS means a level that is 18 decibels lower than the maximum level your system can handle.
In summary, decibels are used to describe differences or changes in level. 0 dB means "no change."
Values in dBFS are used to describe signal levels in comparison with the highest level your system can handle.
Now you're ready for the big time!