Your extension need the following folder and file structur. The composer file optional, but is nice to have ;) and only a small version. For more details about the composer file have a look at the composer page.
More information and details about the configuration options could be found at the TYPO3 CoreApiReference.
your_extension
|- Classes
| |- Domain
| | |- Model
| | | |- Dto
| | | |- EmConfiguration.php
| |- Utility
| |- EmConfigurationUtility.php
|- Resources
| |- Private
| |- Languages
| |- de.locallang_extmng.xlf
| |- locallang_extmng.xlf
|- composer.json
|- ext_conf_template.txt
|- ext_localconf.php