The path starts with the F=ma exam, open to any US high school student. Top scorers are invited to the USAPhO, a much harder free-response exam. The very top performers from USAPhO are invited to a training camp, from which the US Physics Team (and eventually the 5-student IPhO team) is selected.
Most schools have a teacher registered as an exam coordinator — ask your physics teacher if your school participates, or how to register as an independent student if not.