Marta provides a Lua API for writing plugins. Lua is a perfect choice for Marta: it is simple yet powerful, lightweight and mature. See the blog post about why Lua was chosen.
In this page you can find the API reference and tutorials about how to write a plugin.
API reference (2.1)
The API is available under the CC BY-ND 4.0 license.
Here are some tutorials that will help you to understand how to write Marta plugins.