I wouldn't worry about standardized tests just yet but you should probably set a brief 4 year plan, with what prospective SAT subject tests you'd like to take (coinciding with your classes) and the types of ECs you're interested in and the full extent to which you can explore that EC (national awards, leadership, etc).
Also, you can also start looking at summer programs that you'd be interested in. My main problem was procrastination and missing program deadlines that I now regret not being able to apply for.
All in all, it's practical to start early in practice SAT/ACT testing, I took the SAT in 7th grade (got 1600-1700s) through Duke TIP and it was a good experience in how to study and take such a rigorous standardized test. Over 4 years, I've learned how to take these tests and how to do well (in general). However, don't stress yourself at all about SAT/ACT, you still have a long way ahead of you. It'd be more practical to start devoting yourself with ECs and making good connections with teachers/sponsors/supervisors.