By louder it should be assumed that what's really wanted is a "fuller voice." Many inexperienced and/or untrained singers get a rather "thin" voice by trying to do it all in their mouth. A "full voice" depends on getting the whole vocal tract, including the deepest parts of the lungs, into the act.

A good practice would be to join a good (not small church amateur) choir. Even better would be some practice with a small "close-harmony" group like a barbershop quartet. When you "hit the spot" in close harmony, you should "feel the resonance" all the way down to your belly, and the hair on your legs will quiver - accustoming you to what you're looking for.

You might try some "echo chamber" practice - which might be in your shower if the resonance is right, or in something like an empty gymnasium or parking garage. Pick a note in your easy range, and sustain it while you try to feel it "down there." GET IT OUT OF YOUR HEAD and into your chest (heart) by opening things (throat and chest) up as much as possible. You'll feel it inside when you begin to get it.

The muscular action to practice is similar to what you do when the cutie in a bikini walks past on the beach. Chest out (tits up), but don't try too hard at sucking in the belly. Open all of the throat.

Once you get the full chest sonance (full vocal tract all working) you should be able to get "as loud as you want" without blowing out a candle held six inches in front of your mouth, since "loud" requires no more breath than any other noises you make.

Lots of the effectiveness of a "thinnish" voice is wasted in high harmonics of the pitch you're singing, which also tend to be "nasal" and otherwise disagreeable, and using the resonance of the full throat and lung volume will usually also give emphasis to the lower harmonics, giving a more "pleasing," "fuller," and more intelligible voice.

Like anything else, this can be overdone. If you work too hard at it you might "go operatic" in ways that might not be suited to the songs you sing - so if that's applicable, just don't get carried away(?). Loud isn't difficult, and shouldn't really be any harder on your oxygen balance or your vocal chords than weakly squeaking, if you open up right.

John