A box of .535 round ball (for .54cal GPR, my recommendation) (or whatever caliber you choose), a tin of #11 percussion caps, .015" Lubed patches (or different sizes, you have to try a few different sizes to see what your gun likes) , A Cleaning jag/ball seater, some cotton patches for cleaning, and some form of solvent for cleaning between shots, a powder measure, and some FFg black powder or equivalent. Doesn't hurt to get a "range rod" instead of using the pretty wood ram rod that comes with the gun. Wouldn't hurt to get a ball puller too. Thats about all you NEED to go shooting. There is always a ton of other stuff you will accumulate.
"I prefer dangerous freedom over peaceful slavery."
- Thomas Jefferson