The on-line API documentation explains the data that modules expect and return on their APIs.
To connect to the APIs there are several software options:
The Okapi Guide has detailed examples and explanations how to use APIs with command-line tool
The okapi-curl is a shell script that wraps curl.
The okapi-cli is a command-line interface written in Java.
The OkapiCLI.py is a command-line interface written in Python.