There is a priest you can talk to to start the quest for the cure. After talking to him you will be directed to talk to several other people in succession before actually starting to look for the ingredients.
Seeing as you have been told about the gems you have already made it through the bit above, however. You can get them either by stealing them from the Mage's Guild or buy from them. Once you do have them go back to the witch and she will give you a grocery list of ingredients.
- 6 Garlic Cloves
- 2 Bloodgrass shoots
- 5 nightshade leaves
- Argonian Blood
- Ashes of a vampire
Garlic and bloodgrass are found easily. Nightshade can be found between Bravil and Skingrad as well as at the Arcane University. Argonian blood can be had easy as well, just do not get caught trying for it. Any Argorian stupid enough to be in the right place at the wrong time will do.
For the Vampire Dust, however, you must go to Red Water slough and find the vampire Hindaril. make your way through the dungeon here and kill him to get the dust.