Probably has to do somewhat with breathing. Practice filling your lungs, and then keep your chest out, but you can continue to breath. Sing songs out loud doing this. Oprah singers often look large chested, because they actually do this as part as their training to carry notes longer and stronger.
Nerves make breathing more shallow, so conquering nerves and taking some slow, deep breaths will help.
You need to learn to project, always mentally talking to the back row of the room.
Also, certain foods will help. Cold food and water tighten up the throat. Warm food, I find Italian foods, to give me a stronger, deeper voice. Warm tea might work for you? The way to help figure it out in my opinion is again to sing along with some music and see when you feel your voice is much stronger.
You can tape yourself and listen to yourself and practice that way. But a lot of people hate to listen to themselves,