1. This is NOT the internal oven light. Replacing that is a little more involved (here's one guide), and it requires a different replacement light bulb. What we're showing you is the bulb at the bottom of the oven that lights the counter or stovetop beneath the oven.
2. Let's make ordering simple: you can use this bulb or this bulb. These are 12V 20W bulbs as shown in the instructions; they're also the right size with the right base type. You might want to order 2 so you have an extra around for the future.
3. Make sure to wear a glove or otherwise avoid touching the new bulb when putting it into the oven. Oils from your fingers can shorten the life of the bulb.
Alternately, you can use this LED bulb. It has the same base and, although the body's a little larger, it still fit into my oven when I tested it. With the LED, you don't need to worry about touching the bulb. (This will not shorten its life.) And of course it is rated to last far, far longer than a halogen bulb.
Now follow the instructions below and you should have that light working in no time!