If you are singing into a mike while playing a guitar make sure that you can hear your insturment as well as your voice. If you are playing un-rehearsed with a house band of some sort they can lead you badly astray. They may be playing correctly in the way that a song was recorded but maybe not right for you. If you can hear your own guitar it should hold you on key.
If you do not have a built in pick-up make sure that you have a mike for your guitar.
Also bad accustics where you hear the note that you just sang bouncing back as your voice moves to the next note can trash anybody's performance. A good monitor is your best friend.
               Sandy