To use the PGP features of Kmail, you will need to have public-key encryption software such as GNU Privacy Guard or PGP installed and have your keys created. To configure PGP, select File | Settings... and click on the PGP tab. Type in your PGP user identity in the space provided and select any other options that you want before pressing the OK button to save your settings.
Once PGP is configured, you can use it to sign and encrypt messages. To sign an email message, toggle the quill button on the button bar. When you send your message, you will be prompted for your PGP password. The message will then be signed with your PGP signature and sent.
Encrypting a message is also easy. You must have the public key(s) of the recipient(s) of your message in order for encryption to work. Toggle the key button in the button bar to enable encryption. The message will be encrypted once you send it.