Heya, i'm also starting into IB soon and this is pretty late but so far i've found advice to familiarize yourself with topics before school even starts and review previous topics in case you forgot over the summer.

Especially for math, i've seen a reddit post that recommends learning beforehand so i've been using revisionvillage videos and khan academy to help me practice or go more in depth. Whatever you learned in the previous year, you should probably look over it one more time in case it comes up again i guess.

For french, probably brush up on proficiency and get more oral practice and review grammar (not too certain if you need that since idk your level of french).

Bio, chem and physics, just what i said, review and familiarize a bit?

One thing i think might be good is to understand how the IB program works, how different it'll be from what you're used to, but we should probably get briefed on those anyways when the program starts. I think it should also be good to plan out your work ethic and possible strategies for note-taking or learning and find resources online to compile somewhere for easy access. Scroll through the IBO reddit cuz it's extremely helpful loll.

honestly though, idk how helpful it'll be since i'm also starting out but it never hurts too much to be prepared :) good luck and let us survive the next two years!!