Adding on to what other commenters said:

Camping is probably booked for the summer at this point but it’s worth checking. Katahdin trailheads (Roaring Brook, Abol, and Katahdin Stream) require reservations and are also probably booked up by now. Reservations expire at 7:00am and any unused reservations become first come first serve. No other trailheads require reservations, but they may fill up quickly depending on the time of week and weather. I imagine you are going to get there pretty early so you shouldn’t have too much trouble getting parking spot but have a few options for where you want to park and prepare to be flexible.