Comment by omgitstom

This question is a little vague. My assumption is you mean a REST API.

This is a must watch and encapsulates good design and theory: https://www.youtube.com/watch?v=hdSrT4yjS1g

Good API design, if you are trying to learn from zero, comes from learning from good examples.

These are some of my favorites APIs by design

+ Stripe

+ Twilio

+ Slack

+ Stormpath (fd: I work here)

There is a lot of work that goes around the API design to make it a great API (examples, documentation, live samples, etc)