In general, you should dress as if you were going for a hike in the woods: wear sunscren, a hat, and layers to account for variable weather conditions, etc.You might also want to bring a CamelBak or a water bottle, and some snacks.
One key item to wear on the Intermediate and Advanced courses is: long pants or gaitors.
Some people wear ankle-length gaitors to keep gravel and debris out of their shoes while running.
Others wear knee-length gaitors (or pants) to protect their shins from being scratched by brush. Keep in mind that you'll likely be travelling off-trail -- and possibly through thick brush -- if you're on one of the Intermediate or Advanced courses (Orange, Green or Red).