The answers to your questions could take a lot of writing but I'll touch on a few of the high points.
First off, you can use real black powder or the synthetic black powders like Pyrodex, 777 or one of the other powders that are specifically made for muzzleloaders.
Under no circumstance can you use any type of modern "smokeless" powders. Even the slowest burning types can and will blow your gun up.
You will need some #11 percussion caps and some round balls that are slightly smaller than the bore size. If the gun is a .45 caliber, use .440 or .445 diameter balls.
If the gun is a .50 caliber, use .490 or .495 diameter balls.
These balls will be loaded using a cloth patch between them and the bore.
A typical cloth patch MUST be pure cotton. No synthetic man made material will work. The typical patch thickness for your gun will be .015 thick.
You will need to lubricate the patch and there are special lubes that do a good job like "Bore Butter". If you can't find this, Crisco, Olive Oil or spit can be used. Coat the patch lightly on the side that will be against the barrels bore.
Because the ball diameter and twice the patch thickness is larger than the bore of your gun you will find that you will have to use some sort of starter to get the patched ball into the bore.
They sell a "short starter" that has small dowels on it to shove the ball into the bore but if you don't have one of these, a round wooden ball or even a block of wood can be used.
The starting powder load should be about the same as the caliber, that is, a "50" caliber would use about 50 grains of powder. A "45" would use about 45 grains of powder.
A rough guide for the maximum load would be about twice the caliber so a .45 calibers Max would be about 90 grains and a .50 caliber would be about 100 grains.
Black powder is not corrosive until it is fired.
The fouling left will draw moisture out of the air and this fouling will rapidly rust the bore.
Every time you finish shooting your gun you should wash and dry the barrel.
Black powder and the synthetic black powders are all water soluble so simple soap and water is all you need to clean your gun.
This forum has tons of information about shooting and cleaning your gun so if you have the time, read all of the posts you can.
Have fun and remember, we are here to help you so if you have any questions at all feel free to post them.