F-Droid is just a front end for repositories so there are a few ways you can list an app on it - mainly by hosting your own repo or contributing to an existing one.

If you go into F-Droid -> Settings -> Repositories you can see what repos are already configured. Most major repos will have a guide for how to contribute, and this is the F-Droid official repo guide.

Sorry to reply with a link instead of answer, but the knowledge you seek my be here

https://f-droid.org/en/docs/FAQ_-_App_Developers/