Christine is right on. Wear something which makes you look like a trainer, maybe even a shirt with your company logo if you have one.
Good luck with your interview.
I always try to look as professional as possible while still looking the part of a trainer. I always wear long black workout pants (not as fitted as when I teach) and a nice zip up, fitted jacket (not a sweatshirt and not hooded). My hair is usually pulled back in a ponytail, but that’s just me.
It has worked for me so far! Hope this helps.