Unlike conventional incandescent light bulbs, fluorescent lights do not have a glowing filament. Instead, fluorescent lights contain a mercury vapour that gives off light as it is ionised. For the fluorescent to light properly and safely, the process must involve a fluorescent tube, a ballast and a starter. The starter allows the current to flow through the ends of the tube to light the gas within. Once the starter heats up sufficiently, the contact within opens and interrupts the flow of current so the light remains lit with the ballast acting as the current limiter. Most modern ballasts have built-in starters, but many fixtures in the field still require the ballast to be replaced separately. Since starters are self-contained, inexpensive pieces of technology, it is easier to simply replace one rather than try and fix one.
Turn off the power to the circuit that delivers power to the light. If the switch is nearby, it's safe to turn off the switch since you're not actually dealing with any wires, but if the switch is out of your line of vision, play it safe and turn the circuit breaker off.
Remove the fixture cover if it has one. Shop fixtures usually do not, but in-home fluorescent fixtures do in most cases. Use the screwdriver to remove any necessary screws or open any tabs securing the cover in place.
Remove the fluorescent tubes one at a time by turning the tube 1/2 turn until the pins on the end of the tube are aligned with the slot on the socket. Once aligned, simply pull the tube out of the socket. The starter is directly behind the tubes. It's a round cylinder that's sticking out of the fixture. It is usually silver or white and may feature a red reset button on the end.
Press the red reset button if the starter has one and put the tubes back in the light fixture. Turn the circuit back on and test the light to see if it turns on. If it doesn't work, turn the circuit back off, remove the tubes again and continue to the next step.
Grasp the starter with your fingers and give it a 1/4 turn clockwise. This will unseat the two posts on its other end. Once the starter is loosened, simply pull it out.
Install the new starter, reversing the process of removing the original one. Set the two posts in their sockets and give the starter a 1/4 turn counterclockwise until it snaps in place.
Replace the fluorescent tubes and cover lens if the fixture has one and turn the circuit back on.
Turn the light switch on to test the new starter.
If you replace the starter and the tubes and the light still doesn't work, the ballast probably is bad.