Many people speak from their chests by default. If you're serious about singing, however, you should make an effort to sing from your belly. Not only will you increase your volume, but your singing will take on a sweeter, more open tone.
The foundation of singing is learning how to breathe properly. It takes a lot of strain off of your vocal chords, helps control loudness and just in general helps you sound even better than you already do. If you're having a hard time breathing with your diaphragm try lying on the floor. Try to feel how you have so much more control of your breathing now that you're using your abs to push out that air. For a guide to getting started singing with your diaphragm, watch this free vocal lesson.