Firstly, make sure you get over stage fear- this can be done if you believe in yourself and not care about what other people say about you. Confidence is the key to a great performance. A good voice is what you probably already have and so you would not need to worry too much of the quality of your song.
Try to convey a message through your song. When you sing, your voice and expression should be such that people listening to you can interpret the song closest to your interpretation. People should feel the music and feel the essence of the song deep down.
Always have a smile on your face whether on stage or off stage. When a singer walks up to the stage to perform, the audience always has a pre-conceived notion in the minds. If you are generally a grump who does not appreciate advice and criticism from the audience, they will not express it to you either, which will put you and your music at a disadvantage. Nobody would want to have any kind of deals or partnership with someone unpleasant.