I had my first job interview today at a movie theatre. I felt it went really well, I'm 18 with volunteer and professional work experience. In sales, coaching, and maintenance. Right after the interview he said he wanted a second interview tomorrow at 11:45 with the general manager. Is that a good sign?

Well you beat out some applicants but he may have asked 10 people to come back for a second interview for 1 job

From resumes and applications try to screen out the best 10-20% to come in for a face-to-face to see how well the people handle themselves. From them, screen out the best 3 to 10 applicants to speak with the hiring manager.

It isn't a hard and fast rule, but at least you made it to the next step.

There are times where the first interviewer has selected someone for hiring, and the general manager just confirms the selection. It depends upon the number of applicants.

If it goes well then that's usually a good sign, but you may still have to beat out a few other candidates in order to get the job.

It's a sign that you are on the short list. We don't know how many other people got a 2nd interview though do we?

Yes, you would not have been invited for a 2nd interview if they did not like you.

Any invitation back is a good sign.