Does anyone have advice on what to study before joining the Army as a 15T?

No need to study anything. Generally AIT in the Army, well military in general, are designed to bring about high pass rates. (No I'm not talking no SF courses or crazy crap). Just normal high deman MOS.
The most I would do is familiarize yourself with common tools used in aviation. Watch a YouTube video on safety working. Maybe read up on the UH-60. But to stress over this would be a waste of time.
Enjoy the experience, pay attention, and settle for nothing but your best.
Aviation is Keen on safety..safety..attention to detail...and safety. There is a manual for everything. For everything use your manual. To easy.

Not really it’s all in class No home work

You could try finding a -10 manual online either from the aviation museum gift shop or the hangar. You won’t be able to find anything else unless you can access LIW or APD. I’m pretty sure that those need to be accessed from a government network though. I could be mistaken. I’ve never tried from home.

When I first signed up for aviation with the Guard I got some hands on with the aircraft before I went to school and my unit told me “forget everything we’ve taught you when you go”.

Then the school house said “this is the way we have to teach you, but your unit is going to teach you their way”.

I’m a full time technician for the Guard and I have a hard time getting certain things even with my expanded access.