I was going into my junior year of high school and applied to Panera online for a summer job. I was nervous before the interview but I walked in, spent about 40 minutes inside, and it went really well.
I had applied online, and a week later I got a phone call asking to set up an interview. I went in for the interview the next day wearing khakis and a polo (just to look presentable). I was about 10 minutes early. I went inside to the counter and asked for the manager who called me the night before. The cashier offered me a free fountain drink (which I accepted) and I sat down at a table and waited for about 5 minutes before the manager came and sat down. We spent 15 minutes talking, then he said he'd like to do a second interview, so he got his boss and she sat down and interviewed me right after he did.
They were both very nice and very friendly. The manager told me she really liked me and looked at my online application and offered me $9 an hour (I said I wanted $9 to $10 an hour on the application). She gave me information about going to the headquarters for an orientation class (I live near HQ in St. Louis) and then told me to call her when I completed the training session. Overall, very positive experience.
A lot of questions were just asking to specify things that I said in my application and my resume (I brought my resume with me just to be safe)
Other than that, it was just the usual kind of questions you'd expect (why do you want this job, can you handle stress, when are you free)
Don't worry about the interview! Just show a friendly and enthusiastic personality and your genuine interest in the job
~J