When I got the first sword of obedience in the apartment world (it was stuck in the victim, and he was trapped on the ground) I tried using it to put the ghostie back in the ground, but I could not figure out how to do it. Can someone help me, because this ghost keeps killing me...

You have to knock the ghost to the ground before you stab it with the sword. Try using a silver bullet or holy candle to keep him on the ground while you pin him down.

I'd suggest you keep the sword for later, though. You'll wish you had it when you encounter one of the super ghosts.