minor buying a phone

If by "the apple plan" you mean financing the phone over time, you won't be able to do that. You can walk in and pay the full price of the phone in cash, but you can't get credit to buy the phone unless you're over 18 in the US.

If your parents want to finance it, that's up to them. You can not get credit. As a minor, you're not old enough to legally enter into a credit agreement.