As it is a National Park, camping is restricted to designated campgrounds only. I was there a little over a year ago. The campground that I stayed at was clean, but very busy. I was surprised at how busy it was for the end of September / beginning of October. There are some sites that you can reserve, and others that are on a first come first serve basis.
If you know the dates, I would strongly recommend reservations. I would also recommend you take your own firewood as they have been known to run out.