Citing the Enigmail documentation: one of the best options to start with is to send a signed or encrypted email to Adele (email@example.com)
the "Friendly OpenPGP Email Robot". Adele accepts OpenPGP messages and replies in an explanatory way to any kind of OpenPGP messages. Don't forget to attach your own public key if you send your first email to Adele. You can use the menu OpenPGP -> Attach My Public Key for this.