When you're just starting, nerves can be the biggest obstacle - you know when you sing at home it can sound great! But, when you're on a stage your throat muscles can tighten and the wobbles can hit you. Adrenalin can either make or break your performance - remember it isn't just about delivering a song, it's also about engaging your audience. Practice finding your centre, breath steadily, warm up first and if necessary find a private place to run on the spot or jump up and down! Physical activity is the only way to nuke adrenalin.
Tight throat muscles can mean you start on too high a note - make sure you sort out your start note (instrument etc) before you start. If you blow the first line (ie wrong note) begin again!
Avoid beer - it makes you burb - and only have enough alcohol to feel relaxed (if at all!).
Choose a song that you know so well it has become a friend. It's ok to have a cheat sheet in case your fear takes away all memory of words - but don't read it word for word - a glance reference is all.
Imagine your notes are flying from a bow, and you're casting them to the furtherest wall.
Most importantly - smile, have fun!