Where do I start

It depends on whether you play the Java version (i.e. the current default game) or the new version (the one that's currently in a beta branch and needs to be enabled manually)?

To modify the gameplay or to add new content, the game mainly relies on plugins. For the Java version, you can find plugins here: https://forum.rising-world.net/board/32

For the new version, plugins are located here: https://forum.rising-world.net/board/45

(the Plugin API was just released with the recent update for the new version, so there aren't many plugins available yet and some of them are still work-in-progress)

The Plugin API is a lot more powerful for the new version, so probably we will see more plugins which add new content there in the long run.

To install a plugin, just download it and extract it into the plugin folder in the game directory. For the Java version, it's called "plugins" (lowercase), and for the new version, it's called "Plugins" (located in the "_New Version" subfolder)^^