From the onset of Graveyard Keeper, the donkey will be bringing bodies to the graveyard. This allows you to develop the premises, the church, and your job. At some point, however, the donkey will stop doing that. It will only bring you bodies in exchange for 10 carrots and oil the wheels.
The carrots can be bought in The Dead Horse or grown. Growing them is more viable since the donkey will require at least 10 carrots a week.
The exact guide to establishing the farm can be found in the chapter devoted to harvesting.
Getting the oil is much easier, but there's just one way of obtaining it. In order to get the oil, you need to get to Dig. She's in the tent and she sells veggies. The location of her tent is shown above.
After getting the resources, got to the donkey and give them to it. This will make sure the bodies keep coming.