Calm down and just be yourself in the interview, and you will do just fine.
What should you wear?
You should wear the same outfits that other Horton employees wear under their uniforms (be prepared to go to work right after the interview).
Comfortable shoes because you will be on your feet most of the time.
If you are a guy, get a haircut, take a shower, shave, and by all means brush your teeth (there is nothing worse than bad breath),and no colognes. Your clothes should be cleaned and pressed without any rips or tears.
If you are girl,
The same applies dress conservatively. No low cut shirts, no short-short shorts, no flip flops, no flashy jewelry, tone down your make-up, and by all means no perfumes and fragrances.