Turn off the light switch to cut the power while you work.
Position the stepladder below the light fixture. Recruit a friend to hold the ladder while you work.
Twist a screw-on glass cover counterclockwise to remove it and pass it down to your helper. If your fixture has a hinged cover, pull gently on the side opposite the hinges. The cover swings downward.
Unscrew an incandescent bulb from the socket and pass it down to your helper. Halogen and fluorescent bulbs may pull straight out of the socket.
Insert a new bulb appropriate to your particular light fixture.
Screw on the glass cover or raise the hinge mechanism, depending on your type of fixture.