Okay, so if you wanna reset your iPhone or sell it, you gotta turn off Find My iPhone first. It’s actually pretty easy.
Go to Settings, then tap your name at the very top (that’s your Apple ID stuff). After that, tap Find My, then Find My iPhone. You’ll see a switch to turn it off — just flip that off.
It’ll probably ask you to type in your Apple ID password to make sure it’s really you. Once that’s done, Find My iPhone will be off and you’re good to go.