You can find the Pandora app for Apple TV in the App Store. Search for Pandora and click the Get arrow.
You can either sign in with your email, create a new account, or sign in with an activation code.
If you sign in with an activation code, just go to pandora.com/apple from a browser on another device while leaving the activation screen up on your Apple TV. Enter the code you see on the TV, sign into your account, and your stations should appear on your Apple TV screen.
Note: If you're currently using the third generation of the Apple TV or older, you will not be able to use the Pandora app released for the new 4th generation Apple TV. However, you can use Airplay to stream Pandora from your iPhone, iPad or iPod to your Apple TV.