This fall i am going to be a sophomore, and i want to start doing extracurriculars i was wondering if anyone could help me with which extracurriculars would benefit me when applying to colleges in the future if i want to major in mathematics.
In addition to what @IvyLeagueGirl said, I'd recommend participating in AMC competitions and possibly even getting up to AIME level (that would help a lot with elite college admissions).
You could start a math club at your high school, or do multivariable calculus/diff eq/linear algebra at a local community college or nearby state university.
Also if you are interested in applications of mathematics, you could possibly do a research project to enter into a local or regional science fair. Here are some projects from California for inspiration: csef.usc.edu
Definitely try joining your school MathCounts club. Do lots of math competitions and see if you score high in one of them. You could tutor or virtually tutor young kids on basic math. You could join Art of Problem Solving. Look it up. You could participate in a summer program related to math. Join Math Olympiad. The opportunities are endless.
To keep this community safe and supportive:
Aren't any of these extracurricular canceled due to coronavirus such as Math Olympiad or are there online math cmpetitions?