Oxygen! Breath deep, several large ones before you start. Sing as many non-nervous songs as possible first, or warm up your voice back stage to get your voice box warm and working as well as possible before you start.

These things will help solve the physical issues with vocal 'waver' - if the problem is just plain old nerves, then try to chip away at the problem; sing a few lines of the nervous song to a smaller audience, so you are 'trying it out' not performing - sing it often to small, receptive groups and build confidence, and finally keep in the back of your mind that it is important to you that you sing it well enough for people to catch the song, and not be distracted by your nervous symptoms - then 'damn the torpedoes; full speed ahead!"