I will try to make a practical guide as universal as possible for any Linux, Mac and Windows distribution, on this subject, but first of all what is GPG according to Wikipedia:
«GNU Privacy Guard or GPG is an encryption and digital signature tool, which is a replacement for PGP (Pretty Good Privacy) but with the main difference that it is free software licensed under the GPL. GPG uses the IETF standard called OpenPGP. »
Now if you get to work, first of all you need to install Thunderbird and check to have gpg installed (it is installed in most Linux distributions).
For Windows http://www.gpg4win.org/download.html
For Mac https://www.gpgtools.org/
Already installed Thunderbird going to Tools accessories and in the search box they write Enigmail, they give you search and install the plugin. Must reboot Thunderbird for the changes to take effect.
Already with Thunderbird restarted and with the active plugin a new menu will appear in the menu bar, open the new menu OpenPGP and click on setup wizard.
Follow the wizard (or do it manually if you wish), nothing better than some screenshots to explain the procedure.
When you finish making the keys, it is best to upload them to a gpg public key server, don't worry, it's a network and you only have to upload it to a server, the others will be synchronized. For that go to OpenPGP »Key Management in filter by User ID o key, write your email, select where it says Name and in Key server they give Upload public keys, a server asks them, it doesn't really matter which one they give to, and I gave the reason previously.
Well now to send an encrypted email it is necessary to have the recipient's public key. To look for her again OpenPGP »Key Management» Search Keys and there they look for the recipient's email. Now to send encrypted emails: D.
I hope I have made myself understood. To test the configuration there is the email. I expect to receive a lot of encrypted and signed emails. An excellent idea, because we will know if really who wrote the email is the person who signed.
PS: Make a backup copy of your GPG keys because if you lose it you will never be able to recover it. For that OpenPGP »Key Management in filter by User ID o key write your mail and with the secondary button export keys to a file, save it very well.