Install the GNU Privacy Guard implementation of the OpenPGP standard.

Install and configure a messaging client like Thunderbird 
for use with a plugin supporting OpenPGP.

Enigmail can be installed as a plugin for Thunderbird allowing to encrypt, sign and decrypt emails.

Sending emails which have been signed or encrypted will require 

• the generation of a PGP key pair (a public key and a private key)
• the exchange of public keys with recipients of messages
  (for example by organizing a physical meeting) 

Other initiatives allowing to encrypt, sign and decrypt messages from a browser :

• https://www.mailvelope.com
• https://www.mailpile.is/
• https://github.com/google/end-to-end

Riseup has published a pretty list of OpenPGP best practices