Best accuracy will be found with FFG black powder. I haven't found Pyrodex to be nearly as accurate in my .50-caliber CVA Mountain Rifle.
VERY IMPORTANT –– ENSURE THAT RIFLE IS UNLOADED!
You'd be suprised at how many muzzleloading rifles are left loaded, old originals and new ones. Remove the ramrod, put it down the to where it stops. Where the ramrod meets the muzzle, mark that point with a felt pen or a piece of tape. Now, remove the ramrod and lay it alongside the barrel, aligning the mark on the ramrod with the muzzle. If the ramrrod reaches within 1/4 to 1/2 inch of where the barrel meets the tang, then the barrel is empty.
But if there's an inch or more of gap between the end of the barrel and your ramrod, it could very well be loaded. On a clean bore, you can sometimes shine a light down the bore and see a load, or the bottom of the barrel, but not always. It's often tricky to see.
If you have any doubts, take it to a gunsmith. He'll charge you a few bucks, but it's very cheap peace of mind.
Your .58 caliber rifle may take the larger Musket Caps instead of No. 11. Not sure about that.
Hornady and Speer sell .570" lead balls, 50 to a box. Should be easily found at any gun store.
Patch material will vary, according to the rifle's bore size. Well-washed T-shirt material is about as thin as you want to go. If that's too thin, try well-worn jean material. In either case, the material MUST be 100% cotton.
As to lubricant, Crisco works well enough for familiarization. You can always experiment later. Some like Bore Butter, others like to mix equal parts of Crisco and Beeswax.
Myself, I use the 19th century recipe I tweaked a bit years ago, and was subsequently named after me: Gatofeo No. 1 Bullet Lubricant. It's good for all black powder purposes. The recipe is all over the net; you have to make your own, no one makes it commercially.
Yes, if you use patched round balls you'll need a short ball starter. It's easy enough to make one from 1/2" dowel and a wooden ball or scrap of wood. Have one dowel sticking out 1" to start the ball in the muzzle, another 6-8" dowel to push it down the bore a bit. From there, your ramrod takes over. Form a cup in the end of each dowel piece to fit the curvature of the ball.
For familiarization, there's no sense in using heavy loads. I'd suggest you start with 60 to 70 grains of powder under a patched lead ball. This is plenty to put holes in tin cans and informal targets at 25 yards, out to 50. For hunting, you'll use 100 grains or more.
You'll need a means to measure your powder. There are many inexpensive, brass measures to do this. The kind with funnel attached is particularly useful.
Before loading, snap two or three caps on the nipple while holding the muzzle down, a few inches from a grass stem or leaf. This will drive out any contaminating oil and dry the bore. The stem or leaf should move each time you snap a cap, indicating the channel is clear. If you don't see movement of the stem or leaf, you have a problem.
After pouring the powder down the barrel, bounce the rifle's butt on your foot a few times to settle the powder in the bore.
DO NOT measure from a flask full of powder. If there's an ember in the bore, and you pour directly from the powder horn or flask, you'll end holding a hand grenade.
Ensure the patched ball is seated firmly onto the powder, with absolutely no gap between powder and ball. This can be a dangerous condition.
Wear eye protection. Ensure you have eye protection for young shooters too.
Wear hearing protection.
Never let anyone stand to the side of you when firing. Sparks, hot grease, cap fragments and the like can cause injuries. Observers should be behind you.
Keep powder and caps behind you when firing, so sparks from the gun can't reach them.
Bring a rag to keep your hands clean(er). A small container of disposable hand wipes is good.
A plastic fishing box makes a good container to keep everything together, but store the powder and caps separately when not going to or from the range.
Don't forget a proper-fitting screwdriver to adjust the rife's sights.
Discount the slackjaws that will invariably approach you and say, "You know what you should get! You should have a ...."
I wouldn't dream of walking up to Glock owner and telling him he should have a 1911, or a Colt shooter and say he needs a Smith & Wesson or Ruger, but something about black powder attracts these types. Just tell him you got what you wanted and you're happy with it.