There are three essential elements in overcoming nervousness and turning it into a positive. The first of these is knowing your material well. This means thoroughly rehearsing all aspects of each song before you even consider performing them.
If you are unsure of the words, or the notes or timing of either the melody or the accompaniment, it is not surprising that you would be nervous. The more certain you are of these things, the more you are free to concentrate on expressing the meaning of the lyrics and making great music.
The second element is being comfortable with your equipment and your environment. Most public performances involve the use of microphones. Using a microphone will be discussed later in this lesson. When you are on stage, it is important to be comfortable using the microphone and to not be startled by hearing yourself through the PA system or foldback speakers. If possible, have a sound check before members of the public arrive. Most professional ensembles have a thorough sound check in which all the equipment is tested individually and together at least an hour (preferably more) before the show. This allows everybody to become comfortable with the sound of the room as well as the equipment. If you learn a bit about PA systems you can also communicate your requirements and preferences to the person operating the sound system. Talk to the sound engineer and take an interest in the settings used for the various instruments as well as your voice.
The third element is trusting yourself. If you are considering singing in public, you are probably fairly confident that you are making a good sound when you sing and you have probably received compliments from friends as well. In this case, you should be able to sing equally well or better in public, particularly once an audience begins to respond. Your body instinctively knows every aspect of producing a good vocal sound, so it is usually just a case of "letting go" and becoming part of the music. The more you can become the character in each song (like an actor) the more convincing your performance will be and the better you will be able to deliver it.