From Pandora's FAQ
How do I add or delete a station on my iPhone?
To add a station, touch the "+" icon in the upper left of your station list page (accessible from the 'back arrow' icon in the upper left of the screen when Pandora is playing a song), and enter the desired artist name or song title.
To delete a station, swipe your finger over the station name, then touch the red "Delete" option. (To cancel deletion, touch anywhere else on the screen.)