Diagnosis
The bulb is a standard appliance bulb, but accessing it isn't as simple as just unscrewing a cover from inside the cavity. The bulb is housed in its own assembly near the top of the cooking area.
How to Fix It
1. Unplug the microwave. Safety first. 2. Open the door. You'll see one or two screws on the top front edge of the interior, near the light cover. 3. Remove those screws. The light cover (a plastic lens) should slide or hinge down. 4. The bulb is right there. It's likely a 40-watt T7 halogen bulb (sometimes incandescent). Let it cool if it was recently used, then unscrew it counter-clockwise. 5. Replace it with an identical type and wattage bulb. Don't use a higher wattage, as it can damage the socket or wiring.
If That Didn't Work
If you replace the bulb and it still doesn't light, the issue could be the light socket itself (corroded or loose wires) or the control board not sending power. That's less common than a simple burned-out bulb.
Heads Up
Do not, under any circumstances, try to change this bulb while the unit is plugged in. You're working right near live electrical connections. Taking the minute to unplug it is the most important step.