How to open up an iPhone repair business when i'm 15 years old?

I know how to fix only iPhones. I can fix any part of the iPhone. I want to know how am i going to promote the word that i can actually fix the iPhones. I want to be making money with this business. I don't want to open up a store but i can repair it at home.

how will i actually get the message out that i know how to repair the iPhones so that people will come to me?

Tell your friends. Tell them to tell their parents. Tell people at school.

Consider fixing a couple of phones for free, or for just your cost, if their owners will tell people about your work. Make up some business cards that they can hand out to people who have broken phones.

Put some of the cards up in public places - bulletin boards at the entrance to grocery stores, or at other similar places. Or maybe put up small flyers, too.

If it's not hard to do so, consider doing the work at the person's home or place of business. If you can pack up your tools and commonly used parts in a backpack, could you do the work right at someone's kitchen table? That convenience might be worth a few extra dollars to someone. They'd know that you weren't skimming data out of their phone, because it's right there in the kitchen all this time.