Remember that teachers are public speakers. If you're not comfortable speaking in public, you've got to remedy this situation immediately. Teaching is all about standing before groups of people and communicating information to them clearly and effectively. You can't teach if you're paralyzed by fear or you're so shy that you can barely be heard even in the front row. To be a good teacher and a good public speaker, you must speak with a certain amount of power, self-assurance, and confidence.
None of this means that you have to have a booming baritone voice or that you need to overpower your audience with bullying and arrogance. Nothing of the sort! You just have to inspire trust in your students. They have to feel that you're knowledgeable, prepared, organized, and professional. They have to feel that you're a leader and that you know where to take them and the best way to take them there. You'll never inspire trust if you don't trust yourself. Speak up clearly, as if you know what you're talking about (because you really do).
Remember, in any classroom, large or small, some students must sit in the last row or at least in a part of the room that's somewhat far from you — perhaps ten feet away or more. If they can't hear you properly, they'll fidget, talk, draw pictures, and generally zone out, so make an effort to project your voice.
There's a huge difference between projecting and screaming. Projecting means “turning up the volume,” but constant screaming invites chorditis, a medical condition where your vocal cords become so inflamed you temporarily lose your voice. Teachers who can't talk are like fish that can't swim — dead in the water. Speak up, but never thoughtlessly abuse your voice.
Try to relax as much as possible when you teach, because extreme nervousness can increase your heart rate, elevate your blood pressure, and cause your voice to “flutter” in a potentially embarrassing manner. Making eye contact with each student momentarily as you teach can help you focus your thoughts. Scan the room from left to right then back again continually, and even smile courteously as you make eye contact. You'll be surprised to see that many students will smile back, and this can often help you relax.
Also remember, you're a role model for your students, and you must use correct English at all times unless you're making a funny point and your kids know it. For example, if a kid asks a male teacher to grow a beard, the teacher might reply jokingly, “It ain't gonna happen, bub!” But except for such witticisms, stand as a shining beacon of correct English grammar.