Open Script Editor.
Click Window > Library (or press ⌘+⇧+L).
Click the + sign on the upper left.
Choose an application and click “Open”.
If the app is scriptable, it should have a library that you can add to this list. Otherwise, it’ll tell you you can’t script it!
If you can add the app to your Script Editor library, you should also be able to open the app’s script library, which should tell you what objects and methods are available to you to automate the app. Neato!