Even teachers can have problems speaking in front of people. Usually if you have made it that far you don't have too much of a problem with it, but you may still not be as good as you would like to be.
If you are hoping to improve your public speaking skills (not just for teaching), the best thing you can do is over prepare. Over preparing helps you avoid the stress of having to think on the fly when you are in the moment. It can also give you peace of mind in the time before you have to speak.
Making sure you have everything thought out ahead of time will also improve your lessons of course, so there is more than one benefit of being thorough with your preparation.
So, if you are having problems with nerves when you are teaching, plan out every detail you possibly can. Speaking may be a little scary for you, but there are things you can do to make it less difficult.