The best place to start writing plugins for ILIAS is to check out ilias.de->Development->Development Guide->Plugins.
Depending on what exactly you try to accomplish you need a different plugin type. If you would like to insert videos in any page element such as the page editor or the wiki, you probably want to create a Page Component Plugin. If you would like to use it as a kind of module (such as files, folders, tests, etc.) that can be added by clicking "Add new Item", you would need to do a Repository Object Plugin.
In my opinion, creating a Repository Object Plugin is the best way to start writting plugins for ILIAS. Like that you learn the basics of permissions, modules and control flow.
A "Hello World" example of a Repository Object Plugin can be found in the according section at the very bottom ("Example").
You can find many other examples in the plugin database of ilias.de