What topics should I study before going to 35S school?

We used to call you "Spin and grinners" way back when you would rotate analog dials to tune in signals.

What can you do to prepare? Aside from reading up on what SIGINT is* and how it fits within the larger intelligence structure, not much.

Enjoy your time before you go because you won't be doing a lot of studying at night as you won't be able to take any of the material back to your room (aside from very general theoretical stuff) and will be doing late nights in a SCIF.

If you really want to have some 'exciting' reading, you can look through the Soldier's Manual for a 35S* - should help you to get to sleep at night if you have insomnia.
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* SIGINT overview from NSA - https://www.nsa.gov/Signals-Intelligence/Overview/
* STP 34-35S14-SM-TG (Soldier's Manual and Trainer's Guide for Signals Collector/Exploitation Analyst (35S)) - https://armypubs.army.mil/epubs/DR_pubs/DR_c/pdf/web/ARN2778_STP%2034-35S14-SM-TG%20FINAL%20WEB.pdf

The biggest topic for you to study before AIT will be algebra. If your math is weak you will struggle with some of the topics. Also, you might want to have a look at radio wave propagation and have a solid grasp on map reading. But don't be too worried if you don't have this stuff down before you start. It will be covered pretty thoroughly in school.
Something I wish I'd been told when I went to this MOS's precursor AIT (98J EW/SIGINT Noncommunications Interceptor/Analyst) - get plenty of sleep when you can. Some of the topics are dry as dust and you do NOT want to fall asleep in class.

Just do what you're told, when you're told, and how you're told. Keep your mouth shut and your ears open. Use the thing between your ears. Get off the couch, work out, get in some kind of physical shape. They will teach you everything you need to know.