First you need to get control of the files view, e.g. like this:
private FilesController GetFilesController()
Then you need to hook into the event, when a file is selected/selection changed:
GetFilesController().SelectedFilesChanged += MyFilesViewPartControl_SelectedFilesChanged;
Then you can do your desired action in your event handler:
private void MyFilesViewPartControl_SelectedFilesChanged(object sender, EventArgs e)
var editController = SdlTradosStudio.Application.GetController<EditorController>();
// some code to get the selected file/files.
That about what you need?