Shut off the water heater's gas valve. Open a door or window and let the room air out for about 10 minutes to remove gas fumes.
Pull the cover plate off the side of the water heater near the bottom. Use a flash light to locate the pilot light on the inside of the water heater behind the cover plate.
Turn the gas valve back on to the "pilot" position, pushing down on the button and holding it down to get the gas flowing through the tube. Use a spark ignitor to light the pilot light inside the bottom of the water heater, continuing to hold the pilot button down.
Hold the pilot button down for about a minute and slowly let your hand off the button. Verify that the pilot light is still lit and turn the pilot button to the "on" position.
Reattach the cover plate onto the lower section of the water heater.