Follow a gentle warm-up each day and you'll find your lung strength and capacity will increase.
If you can't read music, look out for song books which come with a CD of the music to sing along to. This will help you learn the notes.
Record yourself singing and listen back to yourself - are you in tune? Are you sounding the words clearly?
Be brave. Try singing in front of a friend for feedback.
When you're singing, make sure you have some water handy. Try to cut back the amount of tea, coffee and alcohol you drink - these can all dehydrate you.
Try to avoid smoky environments, colds and infections. If you have a cold, don't sing - rest your voice until you're feeling better. Never sing with a sore throat.
Your voice is like any other muscle - It gets tired, needs exercise and needs rest, especially if you are making it do things it doesn't normally do. If you haven't done much singing before, it's particularly important to make sure you warm-up properly. Never strain your voice. If you feel tired, stop. Don't risk damaging your voice.