Don't try to adjust your volume because you feel the level is off. Usually the worst judge of level is the singer. My hardest job is adjusting levels for my own performance. Don't scream into the microphone, but don't be afraid of it, either. With virtually all brands and types of microphones you'll find in karaoke, the actual element is at the top and not on the side of the head.
All microphones tend to give priority to the loudest sound over the lower sounds.
Knowing this, you'll find that if you sing directly into the top of the microphone and holding it close to your mouth will pay big dividends in your performance. You don't have to kiss it, simply hold it within an inch or so. It increases the chance that the primary and strongest sound will be your voice and not the surrounding noise. It also decreases the chance of feedback. Often feedback happens because the host is trying to compensate for a soft singer or one who is not using the microphone correctly.