What should I say to the employer (manager) that I don't want the job anymore?

Simple. Go to his office in the morning and stand (do not sit- that invites a longer discussion) there and say, "There has been a change in my personal cirstances and while I really appreciate having been given this job, I won't be able to stay. Because it means a lot to me that you thought I'd be good in this position, I wanted to tell you right away so you can find another person to take my place." If he starts bugging you, just tell him you aren't free to discuss the reasons but you know this is what you need to do. End with thanking him for the opportunity, hold out your hand to shake, and leave.