OK let's see. Where do we start. First off you are going to need a range rod and cleaning jag (.50 cal I presume). Give your bore a good cleaning with a degreaser and/or alcohol followed by a gun cleaner and light coat of gun oil. (Be sure to swab the oil out before you shoot.)
Patch worm: Sooner or later you are going to get a patch in your bore and you may as well have this little rod accessory available.
Next you will need 209 primers. You can get the muzzleloader primers or regular shotgun primers.
Powder measurer: Try to find one that adjust in 5 grain increments.
Powder: (depending on you budget, from what I am hearing Blackhorn 209 is the cleanest burning but the most expensive.) Or you can get some Pyrodex RS.
Bullets: I would select a saboted bullet in the 250 AND 300 grain range. You want to see which shoot best as all MLs are different.
Suggested loads: I would start at 80 gr and work up in 10 gr increments to no more than 120 gr and see where you better grouping is. (Note this is by volume not weight)
This should get you started. Welcome to the club.