Behind almost every successful web application, there is an easy-to-use & feature-rich API as they simply help the main application to spread into others & reached by more users.

Also, an API-enabled application can be easily developed further using the API itself.

In order to create an API for your web application, here are 10 tutorials to get you started.

How To Create An API In 10 Tutorials ~> http://www.webresourcesdepot.com/how-to-create-an-api-10-tutorials/
40 Tutorials On How To Create An API ~> http://blog.mashape.com/list-of-40-tutorials-on-how-to-create-an-api/