I agree that it's natural to be nervous. That's ok. If you weren't anxious at all it might indicate not caring. You care how you'll do, therefore there will naturally be nervousness. Already dealing with anxiety the level might be increased. That's ok too. (Btw, I'm 55 and often feel overwhelmed by new or unknown things too. Try not to "should" yourself about age. That is to say there are really no givens, so no need to burden youself with that thinking. Hope that makes sense.)

Is it a super small store? Are you working a department alone or working with your department, but without a trainer at your side?