Wonderful! You are going to love it:)
As to advice - try to plan
what you are going to do and go through it (roughly) the day before (talking to the walls helps!). That should give you some smoothness and confidence
which the trainees are going to appreciate. Also, this way you will get accustomed to your own voice in such situations.
Another thing - if you forget what you were going to do next - improvise
! Everything's fine as long as you seem confident
, just jump to whatever you see fit.
Try to smile and be positive
and relaxed and convey that feeling to the group
Remember that they came because of your expertise and be confident about it.Be natural
, be easy on them and show no hesitation. Talk loudly
(but still, naturally) and give your best!
Hope this helps. It's wonderful to do what you are going to do! Congratulations:)