Here is just a few thoughts to come to mind:
1) Be patient with yourself and your new students; no need to impress anyone
2) Be honest – tell them that you’re excited and nourvous about teaching your first pilates mat class
3) Teach what you know well and keep it basic – later you’ll add your own brilliance
4)Connect with your students – use eye contact, get to know their names, offer hands-on-to enhance their personal connection and experience with the exercise or even breathing, consider teaching closer to your students rather than on a podium or stage
Lastly, I remeber having my first class too… I must have peed (pe-pe) 10x before class just to calm my nerves, bladders tell all!