Take this as a compliment, but I find it sad that many on this board are so impressed with a 16 year old willing to work. Why is it sad? Because it's an anomoly! I can't find high school kids to do barn work for $10/hr. Farmers around here are stopping baling small square bales beacuse they can't get kids as help. Sad commentary but I predict you will do well in your endeavors BECAUSE you are an anomoly and a rarity now days.
Just say what you'll do and do what you say and you'll do fine. As a "marketing" tool and because you have little experience, you may want to consider giving a real low ball "introductory" price on the first cut. Watch your time closely, start to finish, then you can base you actual quote on that. Just make sure they understand what you're doing so they don't think that's going to be the regular price. (If you REALLY think they're serious and you'll get them as a customer, you COULD even offer the first cut free. No lose for them as long as you don't get people just grabbing a free cut!)
They get a break on the first cut, even if they decide not to use you, and you get to give an honest quote without having to guesstimate how long it will take. Worst case scenario is you don't make a whole lot if they don't use you but you gain experience. Best case is you get a new customer and priced yourself fairly. Best of luck!