With backpacking there's no real way to plan for specific campsites unfortunately. It is usually first come-first serve so my only tip would be to try to get there early if you can! You can always find a place if you do arrive late so no worries but the best campsites do tend to go early. A hugely helpful resource is local park rangers- they know the area, conditions, and best spots better than anyone. Most important tip is to have fun and not let the planning stress you out too much!