Hey Matt and welcome,
I’ll start off by saying that you are in the right mind-set, in this program the more you get done ahead of time, the better off you will be.
Now since you have a good amount of time ahead of you, I would recommend starting out with the private (PAR) written exam prep, you have free access to Study Buddy which is also what ATP recommends using during the program. The requirement at ATP is that you have the written exam completed before your first solo flight, which happens about 4-6 weeks in, but obviously if you show up on the first day with the written prep already complete, ready to take the exam, you will have that much more time to focus on other material in preparation for your solo flights.
I personally did not use Study Buddy, so I cannot give you much detail as to how it works, but I have heard that it gets the job done from students of mine.
I have also heard of students coming in with ALL 6 writtens completed before day 1, which will save you a great deal of time during training, but that comes at the cost of spending extra money on testing fees ($150 each) since the exam fees are already included in your program’s tuition and I don’t believe they are refundable.
Either way, don’t study for more than one written at a time.
My suggestion is to stick to the Study Buddy for now and try to get the written out of the way as soon as you start at ATP. Other than that, you can start reviewing the following materials:
From the C172 Training Supplement (part of the training bundle):
-Normal Takeoff and Landing profiles
-Stabilized approach philosophy
-Private In-Flight maneuvers (Clean/Landing Configuration Flows, Steep Turns, Slow Flight, Power On/Off Stalls, Emergency Descents)
From the Pilot’s Handbook of Aeronautical Knowledge (AKA “the PHAK”):
-Chapter 4: Principles of Flight
-Chapter 5: Aerodynamics of Flight
-Chapter 7: Aircraft Systems
-Chapter 10: Weight & Balance
-Chapter 12: Weather Theory
-Chapter 14: Airport Operations
-Chapter 15: Airspace
This material will all be covered during training, so don’t worry if some of the reading doesn’t make complete sense, but try to get a basic understanding of these topics, it will allow your instructor to better utilize the time during ground instruction and make it easier for him to dive deeper into each subject with you.