What do you do if you think there's a ghost in your house?

You should double check if you are sure there's one in your house, talk to them and say you are in no harm and ask them to give you a sign. If they give you a sign call a professional ghost cleanser to cleanse your house.

You might also need to call Paranormal Investigators, or a Psychic Medium to determine whether there is a ghost in your house or not, because some things that seem paranormal are just human paranoia and other electrical problems in the house that seem paranormal. If the investigator/medium confirms there is a ghost in your house, get a medium (or just use the one used in the investigation) to determine whether it is a residual haunting (meaning it doesn't know it's dead, and it doesn't know you're in your house, and it just repeats itself over and over again) or if it is an intelligent haunting (meaning it knows it's dead and it knows your in your house doing whatever). If it is residual, you will just have to deal with it, because it doesn't mean any harm. If it is intelligent, you can get a Christian (or Catholic) Priest to help you cleanse the house and have the ghost move on.