The main expectations for very strong computer science candidates are to get to AP Calculus and beyond, and AP Physics. Preferably Calc BC and Physics C but Calc AB and Physics 1/2 is ok too if that’s all you can do. Having solid programming language experience in Python, Java and/or C++ will help a lot and will add a lot to your resume. Building your own website with some meaningful content will be a good EC. Participating in Hackathons and Math competitions will be of great EC benefit as well. Udemy and Coursera will be your friend when it comes to learning about all kinds of new stuff.
You also can think of mixing you two main interests by looking at majors such as Computational Biology or BioEngineering.