I am trying to get Falian in Morthal cure me of vampirism. I bought a Black Soul Gem from him. Now he tells me it needs to be filled. What does that mean and how do I do it? Thanks.

kill a human to fill the black soul gem normal gem only work on none human black are only for humans

you need a weapon with soul trap enchantment, or the soul trap spell, or you won't be able to fill it.
when you have one of those use them on a human and kill him, the gem will fill

You must cast soultrap on a hostile of any of the playable races.
A bandit will do.
Kill him while the soultrap effect is active and the soul will go into the gem.

Kill a humanoid NPC - any stray Bandit or such will do. You'll need to have the Soul Trap spell and cast it on your victim before killing him/her, or use a weapon with a Soul Trap enchantment.

How/where do you get the Soul Trap spell? Also, what weapons have a Soul Trap enchantment?

You can get the spell from Farengar, or many other mages.

The only unique weapon I can think of that has the enchant is the Mace of Molag Bal. It's pretty common on non-unique weapons, though.

You can get the Steel Battleaxe of Fiery Souls from Ironbind Barrow.

Any of the court mages in the holds should have the spell tome for sale - it's an Apprentice level Conjuration spell, so it'll show up in their inventories early.

There is a soul trap dagger in the Midden at Winterhold College. Stuck in the ground near a skeleton.

Get that Sneak up and you're in business.

I cannot even speak to Failon to try and cure my vampirism, he attacks me on site

FEED. You're @ lvl 4. You need to be fed. Go hunting tonight. Then go see Failon..

I've got 13 filled Black Soul gems. I found like 10 of them already filled with "Grand" souls. Three were filled later with the a**hole du jour.

A few notes about filling soul gems if you're new to this.

If you're currently carrying empty soul gems, simply equip a weapon that has a Soul Trap enchantment and use it to kill an enemy. The enemy's soul will fill the soul gem, assuming the soul gem is big enough to contain the soul.

There are different levels of soul gems from smallest to largest (Petty, Lesser, Common, Greater, Grand, and Black).

If you have a few of each size soul gem on you when you kill an enemy with the Soul Trap enchanted weapon, the enemy's soul will go into the smallest available gem that will contain it.

IMPORTANT!!! This means that if you left the house today with 2 of each soul gem, and then you killed two Mudcrabs, their souls would fill the 2 Petty soul gems. If you then went and killed a third Mudcrab, it's going into the smallest available gem that will contain it, which will be a Lesser soul gem because you're out of Petty gems.

The result will look like this in your inventory: Lesser Soul Gem (Petty). This tells you you have a Lesser Soul Gem filled with a Petty soul. Kind of a waste of space, considering soul gems aren't the easiest thing to come by.

Luckily, there's an easy way to EMPTY a soul gem of the soul that's in it. Simply drop it on the ground, and pick it back up. BE CAREFUL if it's a stolen gem (or has a false "stolen" tag...a glitch in the game). If you make the mistake of dropping a "stolen" gem and picking it back up in front of guards, you may find yourself in a little bit of trouble. lol

As long as you're carrying enough of each gem size, you'll never have to worry much about souls going into the wrong size gem, though it's a good idea to check your inventory every now and then to make sure you're not filling up all the smaller soul gems with low-level creatures.

I think that covered it. I'm probably forgetting something though.

- Dave