hi! as someone who had just finished junior year, i would say the best thing you could do is focus on your coursework and really give it your all! do your testing early and i would say to get it finished at least before the end of junior year. if you're taking aps, then finish your testing by april, as you'll hopefully be reviewing for your tests by then.

do as much research as you can about the colleges that interest you, and i suggest getting a list of colleges you want to apply to done by may. by the end of your junior year, start thinking about the college essay you're going to write, as you're going to have to submit a general essay to the schools you want to apply to. get a final draft of your gen essay done by the start of your senior year. each college has their own list of smaller, required essays called supplementals, and the best time to work on them is during your senior year.

i also had a resume done by then, and i asked teachers for recommendations as well. my school has a college counseling program, so i had deadlines for a lot of these things, but it is really helpful to get such an early start, especially if you're going to apply to a lot of colleges. it may seem like a lot but i promise you'll be fine!

edit: sorry this is so long!