DON'T:

Try to sing louder by forcing your voice - it loses tone control and can damage your throat.

DO:

Learn deep (diaphragm) breathing
Learn how to ground yourself when singing
Learn how to control breathing out at a steady rate - a healthy adult can hold a note for at least 20 seconds.
Go to a voice workshop - voice projectin techniques apply to both speaking and singing. Breathing and projection are both harder to learn from a book.
Relax your body before singing (eg Alexander Technique) - tension and poor posture inhibit projection.

When you are singing without amplification you should aim to be heard when singing 'quietly' and 'loudly'. It is firstly about projection (singing from the diaphragm)- the volume comes from voice control.

Also - enjoy what you are doing - the more relaxed you are the less tense you feel.