Ellemnop,
You must be enrolled and within 30 months of graduation to be accepted to the NUPOC program. You must also have completed two semesters of Calculus and Physics each at a minimum and meet other competitive criteria. You should approach an Officer Recruiter the summer after your Freshman year to submit an application, and you will become eligible in ~December of your Sophomore year. It doesn't hurt to reach out to a recruiter beforehand to express interest, but be aware that you won't actually be able to apply yet.
Best,
Brian