The gun's finish is an awfully personal thing. You'd do fine with a furniture stain, a polyurethane, or just oiling it. You just need *something* to act as a preservative to keep the wood from drying up and cracking. I prefer a BLO-based concoction, personally.
You'll need powder and ball. If you hunt or shoot a lot, you'll get into some of the fancier bullets, but round ball is the easy place to start. Go with the Holy Black, not a substitute. Swiss and Goex are the popular brands, with Swiss being the better of the two. You'll need caps or flints, depending on your lock. It should come with a ramrod, but you'll want a starter. It's just a real short ramrod with a big wooden grip on it that helps you start the ball into the muzzle. You'll get to the point that it is unnecessary, but it's real handy when you're starting out. Patch material is next. You can buy pre-cut patches, or you can make your own. I would just buy a package for now. Get a nipple/vent pick, or make one. It's just like a large sewing needle used to clean out the nipple. A length of stiff wire of a gauge that will fit into the hole will do fine, as will a large needle. Pre-made picks are just easier to hold on to. Round it out with a powder measure, and you're off to the races. You don't, strictly speaking, *need* a powder measure, but it keeps you consistent.
Just remember to clean the hell out of it when you're done. Hot water, and plenty of it. 99% of the fouling is water soluble, and the other 1% will be removed by mechanical action when cleaning. This is the reason (among others) that you should go with real BP. Many of the substitutes are chemically corrosive. Lube BP guns with natural oils and greases, or you'll be learning a recipe for a tar that is far superior to anything you'll find on the road. You can purchase specially-made lubes, or you can dive into the wife's Crisco can. You may want to purchase your own personal can of it, especially if you're going to teach her to shoot the thing. Clean it out, dry it thoroughly, and lube it up. Putting some low heat to the metal is often recommended to speed drying. Sticking the parts in the oven (in a pan) on its lowest setting is a popular way to go. Just be real sure you're only sticking steel parts in there. Brass, wood, and plastic don't need heat drying.
There's a wealth of information in the sticky at the top of this forum about all the other stuff. You might also want to pick up the BSA rifle shooting merit badge book, assuming it still covers muzzleloaders. There isn't anything special in it, but it has (or had) some good pictures.
