Here are some tips about how to overcome stage fright:
Gently hum to steady your voice.
Eat a banana before you sing. It will lower that empty or nauseous feeling in your stomach but won't make you feel too full either.
Chew gum. Chewing gum a little bit to ease the tension in your jaw. Just don't chew gum too long or on an empty stomach or you may upset your digestive system a bit.
Stretch. Stretching your arms, legs, back, and shoulders is another great way to reduce the tension in your body.
Meditate. On the morning before you sing, or even an hour before, take 15 to 20 minutes out of your day to meditate. Find a relatively quiet place where you can take a comfortable seat on the ground. Close your eyes and focus on your breathing as you relax every part of your body.
Set a "stop time" for your anxiety. On the day of your performance, tell yourself that you can allow yourself to be nervous for a certain amount of time, but that after a certain hour -- say, 3:00 PM -- all anxiety will go out the door. Just setting this goal and making this promise to yourself will make it much more likely to happen.
I am a professional singer