The muscles involved in projecting the voice are in the diaphragm. You push the air out from below. You shouldn't feel any exertion in your neck, throat, or jaw, except for forming the right shape for the air to pass through. Practice with a hand placed on your belly so that you can feel the muscles there doing the work, and even give them a little push to teach them how to do it.

Also, it's important to figure out what your range is -- the lowest note you can sing without losing power, and the highest note you can sing without cracking or straining or going off-key. Then practice going up and down in that range. And then make sure you never sing outside of that range. Change key as needed to stay within your range. If your range is limited, then on some songs you may have to use some harmony notes to stay within it.