Candygirl,
I am a corporate trainer (of education...I told someone that once and they thought I meant as in gym. One look at me should have told them that was not true!

) so I spend a large percentage of my day in front of people speaking. A couple of suggestions...
Find a group like toastmasters and join. Toastmasters is an organization that helps people improve their public speaking skills.
If I am feeling particularly nervous before a presentation or a class (and believe me, it still happens, even after doing this for over 10 years!) I do some quick exercises to help release the tention. For example, if I am sitting down I will take my hands, place then under the seat and then pull up as if I am trying to pick my self up will sitting in the chair. If I am in a place where no one can see me I do this shake thing with my upper body where I let my arms hang loose and I move my shoulders back and forth (hard to describe!) I also take in deep breathes through my nose and slowly exhale through my mouth. Usually I close my eyes and visualize myself doing a great job. That helps to calm me.
If looking at the people in the audience makes you never look right at the top of their heads. That way, it looks like you are making eye contact but you are really not. Remember to move your head from time to time or folks will thinkyou are weird for staring at them! LOL I have heard people say to imagine the audience naked, but that has never worked for me! YUCK!
Regarding your hands, I actually don't recommend you put your hands behind your back. It draws your body in and makes you appear smaller and tighter which is typically not a good thing. Plus, for women it usually makes our chest stick out. Look at the weather person on your local news and notice how they stand: feet shoulder width apart (make sure your knees are slightly bent and relaxed, you don't want then to lock up!), arms slightly bent, finger either loosely laced together or fingertips lightly touching. It may feel strange at first, but it really is quite comfortable.
I have a lot more I can share with you, but this is getting long so I'll jsut say one more thing. The best way to get over the nervousness is to practice, practice, practice. Look for small opportunities to stretch yourself and speak in front of others. Even if it is just asking questions in class.