Encryption would be in fact the most secure way of achieving data security. This process encrypts plain text to secrets codes. From top secrets messages to everyday emails you wish to send and can be only be read to intended recipients.
Sites nowadays especially those that process payments (like eCommerce sites) or those that require personal information from users are using SSL encryption (Secure Socket Layer) for establishing an encrypted and secure link between a web server and a browser.
Emails from top officials, government, or private are also using encryption which encrypts their messages which is intended and should be readable only to the specific recipient/s.
The way to encrypt your message or email is to use downloadable software available online. PGP (Pretty Good Privacy) is an encryption program used to encrypt and provide privacy for data communication (such as emails). And if you have knowledge of coding or developing software, you can program your own encryption software. Encrypting messages uses keywords to encrypt each character of a text.
There are also ways decrypt( a way to decode the secret code and revert it back to a normal plain text) an encrypted text. To give one is via a brute force attack, which tries a random keyword to decrypt a secret message. The stronger your encryption method the more it is difficult to decode the secret code using brute force. You will need to use a powerful computing system to try possible keywords used to encrypt the text. However, if you using the simple encryption method which can be easily decrypted, you can just use your personal computer to develop a program that can decode it.