Me and a few friends camped in Yellowstone when we drove cross country after college. This was in the during July so we all packed tshirts and light clothes. We also didn't bring food because we planned on fishing and eating that. The only thing i can tell you is it gets freaking cold up there. I had a really light sleeping bag since it was summer, but on the way up to the mountain it was snowing out. It gets warm during the afternoon though. Definitely pack some heavier clothes for the nite time. We didn't make any reservations either...we got there around 11 pm and nobody was around so we just pulled over and slept. Got woken up at 5 am by some park ranger telling us to move. The second nite we just rented a campground and setup shop.