Take singing lessons with a qualified trainer. Try a classical instructor, no matter what genre of music you sing. Perfect your technique to bring out your best voice and your fullest range. Listen to records of other professional singers you look up to. Don't wear out your voice by singing beyond the range you're capable of.
Pick out or write a song you would like to perform. Work out a set list and know it like the back of your hand. Be flexible and have an alternate set list ready to go in case you feel like switching things up onstage. Rehearse thoroughly, singing one to three hours per day. Drink lots of fluids before your big performance. Do not smoke, drink or do drugs beforehand, as they are bad for your voice. Also avoid air conditioning if possible.
Upon performing, own the stage. Center your body language and keep calm and relaxed. Stand at the center of the stage and have an open stance. Interact with and engage your audience. Keep your eyes open. Believe it or not, shutting your eyes does not communicate strong emotions as well as keeping them open.
Everybody messes up on stage sometimes, no matter how much they practice. Do not let your audience know if you make a mistake. Act as if it's part of the act and laugh if off if necessary. If you get distracted or drift away from the song for a moment, waste no time getting back to the song.