Hi there,
Job interviews are tough, even for seasoned people. It seems to me that you were not ready for that interview you went to.
Showing confidence and basic knowledge of the industry, going reasonably well dressed (as a retail worker would dress, for example) and preferably not sweating would have helped you a lot.
As for people interviewing you, don't be surprised or upset when they ask weird questions: some of those questions are tailored to knock people off their rails, just to see how they react to unexpected situations, for example.
It's all part of the test, and there's nothing informal about it. In my early days of jobseeking, I had interviewers being very friendly to me and making me feel totally at ease, only to pull the rug from under my feet in the end. They're a tricky bunch...
Also, when working in retail you're going to have to make a lot of eye contact with the general public. If you can't handle that during the interview, the person who's interviewing you can pick that up pretty quickly.
Google
"jobs at retail" tips, or "key retail skills", and get yourself ready for your next interview.
Good luck!