Where are information about the API?

I need to use the API to develop my application.

Where can I find information about the API, and example of how to use it ?

You can find resource about the API on the documentation:

And of course, you can find many examples and answers to your questions on the Community website, just using the search tool. A lot of members are using API, ask questions, and share tips and tricks.

First I would ask which version of API he/she would want to find.
Second I would point the location (link) of the API to the user.

Third I would do an example of a certain thing to get/set through the API refering the content.

So, for example, if a user asked something about the 5.9 Rest API I would say:

The API for version 5.9 is found on this location

http://www.bonitasoft.org/docs/javadoc/bpm_engine/5.9/

and

http://www.bonitasoft.org/docs/javadoc/rest/5.9/API/index.html

The content was unavailable but now it's online again! :)

So a simple example using REST API would be:

Pointing to your Bonita server on Tomcat or the "server" that the Studio provides you can easily try a rest client such as Advanced Rest Client (chrome) and configure the http request you would want. See the picture below to see an example:

A rest client is a good way to test the request you want prior to "dive" into a more abstract implementation using a Java or C# way to do the request.

Also, I agree with ttoine, the forum is a great place to make your questions!
Makrea