Your exterior and interior doors have hinges that allow it to swing open and shut. Over long periods of time, these hinges can start to creak and get a little stiff. Once it while, walk through the home with some oil or a lubricating product and put a drop on your hinges to free them up.
There are two methods to fix a squeaky door hinge. The first one is the easiest, as all you have to do is use some WD-40 and spray both sides of the hinge with this lubricant. Make sure you have a dry rag to wipe up and drips from the hinge or door itself. Make sure you get to all three or four hinges on the door.
The other method is more complicated, as you need to remove the pin from the hinge itself. Doing this requires a flat head screw driver and a hammer. Once you have the pin out, then you need some lubricant to coat on the pin. Then you need to reinsert the pin into the hinge.
Here is a quick video from Expert Village on these techniques.