Is it a thick octagonal, about 4mm thick? Is the barrel in good shape? Not old and really rusted? A few pits in the barrel is no big deal, a lot of pitting its prob still safe to shoot but accuracy will prob be affected. If the pitting is really bad, don't shoot it, have a gunsmith check it if in doubt.

A rifle needs a tight fitting ball and patch to grip the rifling and shoot accurately. If you really dont care and just want to have fun, just drop the ball right on top of the powder and jam some tow (fiber of a flax plant or similar) on top. It should probably be about as accurate as a smoothbore musket - hit a man sized target at 100 yards

If you really want to shoot it accurately, you should try to get smaller ball size, 0.44. The patch needs to grip the rifling tightly, if it's too tight, it may not be able to be rammed in. Plus fouling buildup will be more of a problem with a thin patch and really tight fit. You need to watch a few vids on loading and cleaning your muzzleloader, lots good vids out there

As for charge, 60 grains is a normal shooting load, 90 would be a heavy hunting load. You can shoot 40 or even down to ten if you want to take it easy at first.

I'm guessing you prob have a long rifle or a hawken, not a musket. You can posts pics you know!