How to prepare for a potential second interview? (First Job)

I recently went for an interview for an office assistant job that involves marketing and staffing support. I spoke with two people that day, the second one more briefly.

If all goes well I just have to speak with the administrator. (they won't be back until next week)

It sounds like they will make a decision on the spot if I were to come in.

I'm scared. I'm anxious.

This could be my first job! (finally after months!)

How different is the first interview is compared to the second one normally? Unsure what to expect.