How do you know when to take the SAT?
You can study for the ACT test each year of high school in varying degrees of intensity. Your freshman year, you just need to know that the ACT test is out there and to monitor your performance on in-school cumulative assessments.
Sophomore year is about figuring out your baseline score on the ACT test. You can find this out by taking the PreACT or a timed, paper practice test at home.
Junior year is ACT crunch time. To start the year in a good place, set some time aside for test prep the summer before your junior year.