First visit https://wordpress.org/plugins/yourpluginslug to see if your desired slug is in use by an existing plugin. If it exists and you don’t want to change the name, a -2 will automatically be appended when you submit your plugin and it passes review.
Your plugin must be GPLv2 licenced or GPLv2 compatible and under 25mb when zipped.
Log into your wordpress.org account and submit your plugin here.