you need to go out and shoot the shot gun, pattern it. Start with a mild load, let's say 65 to 70 gr of ffg powder in a 12 ga. Load equil volume of powder to shot, ie, measure out your powder, then put the same volume of shot into your powder measure.
Then start patterning. I normally put in the powder, then ram a overpowder card down the bore and seat on top of the powder. Then I ram in 1/2 of a 1/2 inch lubed fiber cusion wad on top of the over powder card.
Note that I lube my own wads using a crisco/bore butter mix. The lube keeps the fouling soft and I don't have to wipe the bore all day long. Some say that using a full 1/2 inch fiber wad can blow through the pattern and create holes. I haven't noticed this with my shotgun however, I need to do a lot more load developement so YMWV.
Then I measure and pour the shot down the bore. I then seat a 1/16 inch overshot card on top of the shot. The shotgun is then primed/capped and fired.
There is a good artical on loading a black powder shotgun that will provide more in depth details then I provided.
Also, if your planning on using steel shot, or any of the other non-toxic shot (beside bismuth) you will probility need to use a hard plastic shot cup in order to not damage the barrels.
I also recommend that you follow the directions that came with the firearm, and not exceed any loads that are maximum for the shotgun. On my shotgun, the max load is 89 grs of powder and 1 1/4 ounce of shot (which is 89 gr btw). I find that I don;t need to load to the max to take ducks, or grouse. Normally a 70 to 80 gr charge with the approiate shot amount works quite well.
Read up on the subject and know what you are doing before you shoot the shotgun. Better yet, find someone in your area to show you what you need to do. I believe that the NRA has instructors throughout the states to help in getting started.
This is what I do with my shotgun. My loads may be fine for your gun, or they may be over max. Be careful and remember that free advise is just that, free. It's your gun and face that will be next to the gun, so again, make sure that you have safe loads in your gun before you fire it.
Hope this helps.