Backpacking trip to Yellowstone

“Planning a backpacking trip in Yellowstone, thinking about 2, 4-5 day hikes or one longer hike and was wondering what trails would be good for this? This will be the first time to Yellowstone. One trail or loop we are considering is from the south entrance of the park area and around Heart Lake we will be going in mid to late august(busy time, but only time we could go). With the backcountry campsites, should we reserve the campsites before we get to Yellowstone? Any and all information is greatly appreciated. Thanks”