How would I handle this situation?

I am 16 years old and about to begin my first job. The employer who contacted me told me I would be starting next saturday. Caught in the moment I failed to realize my ACT was scheduled to take place the same day and cannot be rescheduled or manipulated in anyway. How should I handle this with my employer? I am going to an orientation 3 days before the saturday.. would that be a good time to tell them or should I make a phone call before then? If so, what to say?! Any help is greatly appreciated..