I will catch it from some people here, but you can start out with a caliber amount of powder, either FFg or FFFg powder. Only your gun can tell you what it likes by way of powder. I have had large bore guns prefer using FFFg, and I have had small bore guns prefer using FFg. It really depends on the gun.
It will help if you buy Dutch Schoultz Black Powder Accuracy System. For $15.00, he will teach you how to work up a load for any gun, and how to " read " the spent patches.
http://www.blackpowderrifleaccuracy.com/
In addition, you need to know that the Maximum Efficient load of powder your barrel will burn is 11.5 grains of powder per cubic inch of bore. You can put more powder in the barrel, but it will begin burning outside the muzzle of the gun. You can get more velocity with more powder, but its hard on the action, and stock, the barrel harmonics make it more difficult to find a load that will shoot consistently well with the greater charges of powder, and its harder to NOT burn your patches with heavy powder charges. I won't mention the increased recoil you have to deal with.
You have to deal with the sound barrier, much more so than when shooting modern bullets in cartridge guns. If shooting a PRB, it usually doesn't make much sense to get it leaving the muzzle at much over the Speed of sound( 1135-1150 fps) because the ball loses about 25% of its velocity in the first 50 yards.( 40% in the first 100 yds.)
Match shooters, shooting short matches, will either use a very small bore gun and fire a small caliber ball at near 2,000 fps., to reach the target before it drops down into the transonic zone, Or they use a large caliber ball and keep the velocity close to but under the Sound Barrier. Those are not generally concerns for modern smokeless powder reloaders.
See if you can pick up a copy of the Lyman Black Powder Manual, Second Edition. It contains a lot of loading data that is missing from the first edition. It should prove invaluable to you.
If and when you decide on a particular caliber, we can give you more detailed advice on load development for that gun and caliber. :thumbsup: