High School Students! Yes, this includes freshmen and sophomores!
As soon as possible. Meaning the summer before your freshman year is not too soon. Register at College Board to explore colleges and universities and trade schools without any commitment on your part.
The summer before your freshman and sophomore years – Get familiar with the PSAT format. Don’t wait until October when students take the practice tests at school. Take advantage of College Board’s short practice tests on your own time.
Take a look at PrepScholars advice, too. While friends may say you don’t need to study for the PSAT or SAT, you do need to be familiar with the format.
The summer before your junior year – Register at Khan Academy’s Official SAT Practice. I hope you’re familiar with Khan Academy’s offerings in any subject and at any level.
Good luck!