Where are some resources for plugin development?

I checked in the wiki, and in the documentation, but I couldn’t find much on creating plugins there.

I want to create a plugin that lets the logged in user upload an mp3 file or video, and enter details about the file into a new table I will create in the modx database.

That would be a module (to work in the manager) or a snippet (to work in the front end). A plug-in is more for performing functions based on events and not really for stuff like data entry (although you could use a plug-in to render a form and add it to the manager interface in the same fashion that ManageManager plugin works).

Modules are moderately well documented in the documentation. Basically there is not much to know other than it works like basic php structured application but you can have access to the modx api.