Locate the source of the squeak by slowly opening and closing the door, paying attention to the sound. Most often the door has shifted slightly, causing one of the hinges to rub more than it used to when the door was installed.
Lift the door slightly to see if there is any play in the hinges. Tighten all the loose screws using a screwdriver.
Close the door all the way and see if the squeak is caused by the door rubbing against the door frame as it opens. File or sand down the door until it closes and opens without rubbing. Usually, you will see a discoloration on the wood where it was rubbing.
Lubricate all the hinges with silicone spray by closing the door and lifting out the hinge pin. Spray a small amount of silicone into the center opening and onto the hinge pin. Slide the hinge pin back in place. Wipe off any excess silicone with a paper towel.