Big project to tackle by yourself. To be safe about it your going to need a few hands and a dozer or hoe and some scaffolding or better yet a scissor lift. I would start with the outer boards. Strip them off and either sell if possible or cut them up nails and all for the woodburner. You can magnet them out later. Next thing I would do is start pulling the corner beams out with the heavy equipment and try to colapse it on itself. Strip the roof for sheet iron. Try to market those beams as they are worth $$$ in the right circles. Firewood the rest. I would not tackle it for free. I would get all materials and a flat price. Call around a few places that do that type of work for a starting point. You got to go into something like this knowing it's not like knocking a industrial building. No #1 iron here so you got to market the weathered wood and beams hard as your flat rate might just cover your equipment rental and labor. Firewood is a last resort as that's the smallest profit in the thing.

If you get the gig, rent a metal detector and do a little prospecting. Never know as those old school barns were good for a cache now and then.