As a first step, you can practice some simple speeches in front of a mirror.
Practicing is great, but you need to be careful that your speech doesn't then sound over-rehearsed.
As far as relaxing on stage is concerned, the most important thing is to BREATHE!!! Take a few long deep breaths before you walk onto the stage, and make sure you keep breathing properly throughout your presentation. Ensure that your presentation has regular spaces in which to breathe.
Also, regardless of how many people you are speaking to, simply imagine that you are only speaking to a few. Don't allow this to make you speak too quietly or to forget there are people at the back of the room, but simply imagine that there are fewer people in the room than there are.
Do not rush! You may want your presentation to be over, but take your time. Speak in the same way that you would want someone else to speak if you were listening to them.
But, and I go back to my first point, BREATHE! It makes a lot of difference.