This depends on which form you're customizing. If you're customizing the add/edit dataset form, you need to write an IDatasetForm extension. If it's the group form, you want an IGroupForm extension. There's no plugin interface for customizing the organization form yet, unfortunately.
To see how to write a CKAN extension, checkout the new writing extensions tutorial (currently in development, feedback appreciated), and also take a look at the example_idatasetform extension that comes with CKAN.