Hi, Quiet voiced singer. There are obviously many answers and numerous suggestions appropriate to your dillema. Lotsa good advice here, but I'll throw one more thought into the mix.

Do you have a church in your area? Ask the head honcho (sorry, my agnosticism showing through) if you can come in once a week when it's empty, go to the front, and sing your head off for 15 minutes. The resonance will not only be fun (and make you feel powerful!) but you'll learn to control your voice at higher volume. I've done this (did it again last year at the Washington Getaway) and it works.

By the way, I totally agree with Alice...Some vocal instruction can really help...and will NOT turn you into an opera singer (unless you're aiming that way in the first place).....better breathing gives you more volume.

Cheers

Rick