There are books available that will give you some of the fundamental do's and don't on songwriting. I don't know of any offhand, but I've seen them at Borders or Barnes & Noble.

As for singing, I'm no singer but I know one thing you should avoid doing. When you practice, if you sing through a microphone don't use any effects, such as reverb. I see too many singers who rely on reverb to "make them sound good." If you sound good without any effects, you will sound that much better when you add them when you perform.