I talk for a living. It takes time to improve. Some quick and dirty tips (sorry, english not my first language):

- Relax and meditate

- Practice a LOT. Everyday. Even two minutes helps.

- Learn to breath properly

- Record yourself speaking aloud at home with your phone. Make notice of all the nuances of the voice

- Prepare your material very well

- Visualize you are explaining the stuff to a friend at home. It will help you sound more natural.

Getting nervous is normal and part of the game.