Welcome to the Forum.
If my information is correct this is a .50 cal flintlock. It has a 1:48 twist barrel making it suitable for shooting round balls or light to medium weight bullets.
If you want to shoot patched roundballs a .490 diameter ball patched with a .015-.018 thick cotton patch will work very well.
Flintlocks will only work well with real black powder.
None of the substitute black powders will give enough flash in the pan to ignite the main powder charge in the barrel. Additionally, none of the substitute black powders will ignite from the flash of even real black powder in the pan because all of them have a MUCH higher ignition temperature than real black powder.
Real black powder in small quantities is sometimes difficult to get because of the Federal and State rules that the dealers have to live with.
A gun store cannot for instance sit a can of real black powder on his store shelves. You have to ask him/her if they carry it.
Your gun will work fine with 2Fg or 3Fg powder but if you have a choice buy the 3Fg. It will work better in the pan for priming.
Your powder load will vary depending on the gun as much as anything else. Muzzleloading rifles all have one or two powder loads that they work best with and this can vary between identical guns.
A good load to start off with would be about 60 grains of powder down the barrel. After shooting this load try increasing the load by 5 or 10 grain increments. Somewhere between 60 and 100 grains you will find a load that is the most accurate.
If you choose to shoot the patched roundballs you will find that a .490 diameter ball placed in a .015 thick cotton patch lubricated with Crisco, olive oil or something similar will still need something to get it started into the bore.
There is a device that looks like a round wooden ball with two dowels sticking out of it.
You place the short dowel on the ball and give the ball a whack to start the ball/patch into the bore. Then use the longer dowel to push it down about 2-3 inches. From there on you use the ramrod in about 8 inch steps to push the load down on top of the powder.
When priming the pan, fill it about 3/4 full making sure the vent hole in the side of the barrel is not covered.
Close the frizzen (the thing the flint hits), cock the gun, aim and fire.
If you have any questions about shooting your flintlock you can post them in the Flintlock section of the forum for the best answers from our members who shoot Flintlocks.
Have fun.