Tess, The 51 is a great revolver. I just bought one right after Christmas but it's not one I would recomend for a first time BP revolver.The reason is that the open tops are a little harder to tune and can be a pain in the butt to shoot when the caps fall into the works and lock things up.
If you start with an all steel 1858 I feel that you will really enjoy it. Cabelas has about the best price on them and buy the steel , not the brass frame.
When you buy it grt the starter kit they sell with it. That will give you a powder flask, 100 .451 balls, cleaning kit, a 100 wonder wads and so on. All you will need is your powder and caps.
Real Goex or Elephant or Swiss or any of the real powders are becoming really hard to find. Check your yellow oages or do a Google search for muzzle loader shops in your area.Most of the good small shops still carry it.Your next best bet would be to get some Hodges Pyrodex "P". It's the closest powder you will find to the real thing and it loads volume for volume the same as the true fffg black powders.You will want a good adjustable powder measure with a small funnel on it. You will also need some Reminton #10 caps. Some seem to like the #11 best but I think they are a little to large. It's just a matter of choice. A capper is hard to find for a revolver so most just use there fingers to just start the cap and then use a piece of 3/8"x6 doll rod with a piece of round leather glued on the ends to shove the cap on snug to the nipple.
To start out with yu want to strip your gun down to nothing and get to know it's working parts and clean it of all grease and oil that it comes packed in.Clean the barrel really well ! Check all the parts for burs or snags( Anything that looks or feels like a hang nail,Lol) and smooth them up with a file or stone and sand paper. You don't really want to remove any metal, just smooth up all the parts. You will get into a lot of others things to do to it as you go along but for now we just want to get you shooten
I would use 20g. of Pyrodex"P" or Goex fffg. You can work your way up from there until you find what works best for you.
Set the cylinder on your bench or table and charge one cylinder with the powder and then slide a wonder wad over the mouth of the chamber and push the wad evenly down on the powder with the stick you made to seat the caps. Do each chamber that way and then replace the cylinder back in the revolver with the hammer on half cock , slide the cylinder pin into the cylinder hole and then load each chamber with a ball useing the rammer on the revolver. I then Like to use a little grease over the balls just tohelp keep things lubed and running well. Place the hammer down on one of the cylinder notches for safe carrying and your ready to go shoot. Have fun! Mike