How to make an Alexa skill that looks up something based on input?

Question

I'm trying to build an Alexa skill that mimics the functionality of the NPR skill. For example, assuming that the invocation name is "NPR", the dialogue would look something like this:

• User: Alexa, open NPR
• Alexa: 'Welcome, please tell me a ZIP code or city'
• User: 44691
• Alexa: In that ZIP code, there are two radio stations: radio1 and radio2 (example radio stations). Which one would you like to play?
• User: radio1
• (At this point, Alexa plays radio1)

So far, I have two slots {zip} and {city}. I have also made an intent named "ZiporCityIntent" that is supposed to catch the ZIP code or city that the user says. However, I am lost at this point and am not sure how to proceed. My goal is as follows:

1. To have Alexa take in the ZIP code or city input
2. To have Alexa look up the available radio stations at that ZIP code or city and list them to the user
3. To play the radio station specified by the user (from the available radio stations)

I would appreciate any advice or guidance. Thank you for your time.