Look at your own level of interest. We don’t want you to burn out by senior year, but in general, junior year is a good starting point. Of course, if you need to get started sooner to calm anxiety levels, then you should, but make it more like a check-in-once-a-month kind thing.
Keep in mind that you need to be aware of subject tests, PSAT for National Merit, and taking a comfortable but challenging course load. Also get in involved in personally meaningful activities, beginning your freshman year.