Just be nice, be yourself. Type something up, maybe have your parents help on that or something(not that I'm trying to demean your talent) and I'm sure that things will go your way.
Heck, if I owned a farm and some kid knocked on my door asking if he could metal detect I'd be like sure go ahead, just watch out for the cows though lolol. Just make sure you don't dig into any sprinkler lines as those farmers have MASSIVE sprinklers that cost THOUSANDS.
Be prepared for any answer. Also remember that a farmer's land is an extension of their home. It's their way of life and how they support their life. Some people might not be ready for you to start digging. Good luck and keep us updated you youngster!!!