It is a job interview, look professional. I wore a suit (black) with pants, but I think most women wear skirt suits. Since it is getting hoter, I'm sure a nice blouse and dress slacks/skirt would work. (No tank tops, see-through, tight or jeans)
Relax. Interviewers expect the interviewees to be a little stressed out and fumble a bit. Take some time to go back over some of your nursing highlights: positive and negative experiences and your reactions to them. Think about why you are interested in WHC and what interests you for the unit(s) you're applying for. I got asked a handful of simple nursing scenarios like "what would you do if your pt..." Nothing complex. If you get a chance to meet people on the floor, do it. Think about questions for your future coworkers (unit turnover, support from other nurses, etc). Make sure you have questions.
The process is usually meet with the HR person and they will take you to the unit to meet with the unit director (or someone else). Then you'll be brought back to HR to finish any paperwork.
I forget, I think I was done everything for my interview within about 3 hours. I heard back a few days later.