Create methods and classes to control/use your plugin and make them easily available for other plugins to use. Make sure you document those classes and methods as well.
@M4UR0 https://bukkit.org/threads/making-an-api-then-referencing-it-in-another-plugin.138080/ This link may help.
@M4UR0 There is no particular way to make an API. This could be in the form of a class that has static methods, or like what Bukkit does, and have multiple interfaces/abstract classes for developers to implement (i.e. CraftBukkit implements the Bukkit API, Glowstone implements the Bukkit API, SpongeForge implements SpongeAPI etc...), to libraries such as the Apache Commons project. APIs allow other developers to access aspects of your work, without having to touch your work. If anyone wants to clarify, add-on, correct anything I said above, feel free. (@teej107)