To me it sounds like you aren't actually nervous about the material but rather the concept of taking on new trainees (students). Don't worry, everyone has a first day! You can only get experience one way...
While I'm not a trainer, I am a professor so I thought you might like to hear probably the two biggest things that have helped me in my teaching thus far, one practical and one philosophical.
Practical - Plan your class and teach your plan. Corollary: have a backup plan and a backup-backup plan. What are you going to teach that day? Have a goal for the day (maybe a new skill, maybe working a certain energy pathway, whatever) then teach to that goal. The key here is an acronym 'SWBAT' or Students Will Be Able To. What will each trainee be able to do at the end of the day? Plan your class with that goal in mind. And plan out details, especially your first couple times! Think 'ok 5 min for warmup, then 10 min for this drill, then...' not 'right, today we'll work on some squats then a snatch test or something'. Use specifics! Also, regarding the Corollary, What do you do if your new trainees have trouble with basic activities like getting up off the ground or picking up something from the floor? What about if you have some RKC certified super-athlete? Have backup plans for different things that could arise.
Philosophical - You don't need to be the expert in everything to teach. You only need to know more than those you are teaching. That being said, you should try to be that expert, but it takes time. Basically, as long as you are providing 'value' for your trainees (and not getting them injured) then you're a trainer. I spent a whole summer teaching windsurfing as an instructor, learning what I was teaching to my students only the day before each class. Was I an Olympic-level windsurfer? No. Was I able to teach them to windsurf? Of course!
I hope this helps and tells us all how it goes.