I just got it form the deadric quest and I remember it saying that it is used as a soul gem. I can't seem to figure out how to use it to enchant things... Help please.

It's used the same as a normal soul gem, the only difference is that it don't get destroyed when you use it to enchant something. Also you don't say if you have the white or black version, white version can only capture lesser souls and black can capture human souls.

Ok, to fill a soul gem you need to cast soultrap on a enemy and have it active when they die,

You can cast it on an enemy by either casting the spell on them or hitting them with a weapon enchanted with soul trap. Each of these will have a duration highlighted in the spell or weapon description.

Once this is done and you have a Filled soulgem, you have 2 options

1. fill an already enchanted items
2. create a new enchanted item.

for the first you highlight the item in your inventary press:

Xbox - Right bumper button
PS3 - R button
PC - T

This will bring up the soulgems you can fill the item with, just pick the one you want to use.

For the second you need to visit an enchanting table, they look similar to an alchemy table and are often in the same places as mages, so city castles or the college are good places to start, or even are a buyable upgrade for some of the house.

Once you activate the table you have the options to pick an Item, Spell, Gem. You need to choose one in all these 3 options to enchant.

Also you need to have the enchantment in your list of enchantments, you add to this list by de-enchanting items as you play the game.

It is used like a grand soul gem. However it won't fill if you have other grand empty soul gems in your inventory. It can store one at a time. It is the only soul gem that was capable of storing Vivec.