In a pinch you can use 2Fg or 3Fg, but because 4Fg has the smallest grains it is probably the best for the beginner. Then you'll need some 2Fg or 3Fg for the main powder charge. You don't need a horn or flask at first, but you will need an adjustable powder measure. Just pour from the can into the measure, replace the cap on the powder, and pour the measured powder down the barrel. You will also need patches and patch lube for firing and a round ball made from pure lead.
Several companies offer lead round balls that are swaged, or pressure formed, instead of being cast.
They are a bit more expensive but worth it, especially if you're just starting out. A short-starter will help with the loading process, and most muzzleloaders have a loading rod, often called a ramrod by Hollywood types or a wiping stick by old timers, attached. For a flintlock, you will need a gun flint of the proper size and a piece of lead or leather to help hold the flint in the cock. A beginner's book is a good idea as it will have snapshots or sketches of the wrapping techniques. There are several good ones on the market like the Black Powder Digest.