To get an idea of how much air you should have in your lungs when singing, exhale all of your breath. Now, think of yourself at the end of a sentence where you have used all your breath and you want to say one more thing before your friend walks away, like you are going to say, "Oh, and one more thing..." That is all the air you should have in your lungs and you need to control that amount of air. Any more air in your lungs than that while singing will only make it difficult to sing with any power.