Bundle ID is the name of your package. It must remain the same for all future updates too, otherwise you wont be able to update your game. Common practice to name bundles on ALL formats is com.yourgamecompanyname.nameofyourgame
As per "how can I get one": It is a free text field, you make up your own. And that's the reason for the format mentioned by @jigglybean, that way you can be sure that your name is unique. For example if your game called "Angry Birds" your id will be "com.dino222.angrybirds" while mine with the same name will be "com.trudnai.angrybirds"...
