You can find the Pandora app in the App Store on your iPhone. Search for Pandora and tapInstall.
You can also go to pandora.com on your phone or tablet's web browser, scroll down and select Get the App to install the app.
Downloading any app from the App Store will require that you enter the password for your Apple ID. Keep in mind that the email address and password for your Apple ID account may be different than your Pandora account.
Also, please be aware that Pandora requires iOS 12 or higher. If you are running iOS 11 or lower, you will need to update your iOS version.