From now on when you and your contacts use the latest version of the app, every call you make, and every message, photo, video, file, and voice message you send, is end-to-end encrypted by default, including group chats.
"The idea is simple: when you send a message, the only person who can read it is the person or group chat that you send that message to. No one can see inside that message. Not cybercriminals. Not hackers. Not oppressive regimes. Not even us. End-to-end encryption helps make communication via WhatsApp private – sort of like a face-to-face conversation."
You can learn more about how end to end encryption works by clicking here. You don't need to do anything to encrypt your messages. If you have the latest version of the app, your messages are encrypted all the time.
You can download WhatsApp Messenger from the App Store for free.