Dan's right with the clothing; something black on your bottom and something relatively plain on your top. Sandals are always a good choice if you haven't got some little closed shoes; but go for 'dressy' rather than 'I'm going to the beach
'.
It's not a girly thing to care what you wear. What you wear reflects how you present yourself. How you present yourself reflects not only the work you do, but the person you are.
Try to match your employer's register (to use an English word) when you answer his/her questions -- being enthusiastic is important, but being overly enthusiastic can make people look... er. Dumb? I've only personally conducted one set of interviewing, but I was much more impressed with people who seemed to be able to cope with how I was talking with them.
Be sure to smile and laugh (when needed), sit up straight (but be comfortable), and try to ask questions (when appropriate). I think a big thing is moderation of everything; you want to appear fun and bouncy, but also bright and hard-working.
Most important is that you are a quick learner and willing to learn new things. I'd also make sure you express that the legal career is appealing to you, and the job would be great because it would give you a little insight into the career.
Mention that UWS has a very flexible timetable (I don't know if it actually does...) so that you can arrange your work, study and cocurricular (which you have a good amount of
) activities in a way which you will be able to work consistently for them.