They will either mail you an acceptance letter or more likely send you something electronically or have you visit their website.
Some schools will also call.
If you don’t get accepted and put you on the waitlist make sure you find out the timetable for when the move to their wait pool. I know a lot of families who held out for “THE” school and were not eventually accepted. They had to go to public or a private out of their area and/or price range.