Welcome to Target and welcome to The Break Room.
Orientation is nothing to sweat. Depending on your store, they might require you to wear red and khaki to your orientation. At my store they did, however some stores allow you to dress freely (so long as it's neat) for orientation. You'll fill out some paperwork, watch some videos that go over a few Target basics, and probably get a tour of the store. Orientation day is meant to give you a very generic overview of the entire store. Training for your specific job will begin on your first day of work after orientation.
Enjoy the ride.