Get Signal. Their rolling SSL encryption method is one of very few methods that have no backdoor for the NSA, nor is it prone to man-in-the-middle attack. In order to intercept messages, EVERY SINGLE message sent between peers would be required, from the very beginning of the session (first message with another user or group), as it maintains an encryption chain, requiring the previous message history to decrypt the next message. The only way to achieve that would be to use a man-in-the-middle attack where a third party sits between you and the other end, pretending to be them to you, and you to them, from the beginning. Fortunately, you have only to verify the signature of the users you communicate with (a feature of Signal) before you start messaging the first time to ensure there is no middle-man, because if there was, the signatures could not match. If your signatures match, then you can guarantee no one can intercept your messages.

There is one caveat... malware on your phone can read the messages after they're decrypted on your device. So if you are using Android (although I don't recommend it, since google is openly anti-privacy), make sure you take the necessary measures to block google's and other's malware and data-collection. To date, I don't know of any cases where Signal messages have been compromised on an iOS device.