First contact

Deserialization tutorial
Contains a detailed guide on how to parse a JSON document using the library.
Serialization tutorial
Contains a detailed guide on how to generate a JSON document using the library.

Everyday use

API Reference
Contains a detailed description of each function of the library.
Examples
Contains complete programs that shows how to use ArduinoJson in various situations.

Advanced

Upgrading from version 5
How to upgrade code written for ArduinoJson 5 to use ArduinoJson 6.