A squeaky door in your home is most often caused by dirty hinges. You could replace the hinges for around $15 per door, or spend around $10 and fix all the doors in your house.
Hinges squeak when there is friction between either the half of the hinge attached to the door and the half attached to the door jam, or between the hinge and the hinge pin. This instructable will only address the latter issue. If the hinge halves are rubbing against each other, it's probably because the top and bottom hinges aren't aligned and you probably are having much bigger problems with your door than just squeaking, like not closing properly.
Step 1: Tools
The tool list is pretty short. You will need: