Speaking to Father Aereck
Speak to Father Aereck whom is located in the Lumbridge Church and tell him you are in search of a quest. Father Aereck has an issue with a ghost in the graveyard crypt, he would like you to get rid of it. He will tell you about Father Urhney, and about how you will need his help.
Head on over to Father Urhney by going out of the Church and heading south. From there you will want to head south-west until you reach his little shack in the middle of the swamp. Once you talk to him, he will be in a bit of a grumpy mood. Tell him that you were sent by Father Aereck and that there is a ghost haunting the crypt. Father Urhney can't go and help you, but he gives you an amulet you can use to speak to the ghost.
Speaking to the Ghost
Put on the amulet and open the coffin. The ghost will come out; speak to him and ask what the issue is. He thinks the issue is that he lost his head. He believes his head is back at the mining spot south of the graveyard.
Head south to the mining area and search the rock pile just to the east of the area. You will grab the skull and a Skeleton walock (level 11) will attack you. Take the skull back to the ghost and speak to him. He will tell you to place it back in the coffin. Make sure the coffin is open, and then use the skull on it.
Congratulations! Quest Complete!