K...speaking from experience here. I was on the show about 14 years ago....even kissed Bob Barker on the cheek! LOL!
They interview everyone that goes into the studio. They ask you your name, what you do for a living, etc. They are looking for people with lots of personality because that's what makes good television. Most people make their own shirts. If you watch a couple of shows you'll see the wide array of shirts that people wear. A lot of them have little catch phrases on them.
I went back in April for a friends birthday. None of us got chosen but it was still fun. I was never really sure about Drew. He seems kinda boring to me. But, during commercials he talks to the audience and he is hilarious! Definitely worth it to see him. It was more like a stand up comedy act. LOL!
Have fun!