As for modern test prep:
Some schools will give a practice for the P(ractice)SAT in 10th. If your school offers, fine, if not, no biggie, IMO.
Then some summer self-study/practice tests leading into 11th, then the PSAT in fall of 11th, then take the ACT/SAT as many times as you want, starting around then.
Our kids took the PSAT but never really did the “should I take the SAT or ACT” thing - we’re in Missouri, which is basically an ACT state. Among other things, our state offers a scholarship (Brightflight) to high test-scoring kids who go to college in-state (public or private), and the structure of how they set the score thresholds means it’s easier to hit the ACT benchmark (though in theory a high enough SAT score can work).