If you want to send messages without worrying that other people might be poking around in the texts you're sending, you should be using an encrypted messaging service.
There's never a true guarantee that what you're doing writing isn't going to be viewed -- often our own mistakes can lead to vulnerabilities that leave us wide open, but using encrypted messages is certainly a step in the right direction.
If you're unfamiliar, encryption helps jumble the content of a message into random data until it's received on the other end and the original message is compiled back together again. This means if anyone intercepts the message -- a hacker for example -- it's mostly just jumbled characters and symbols. Keep in mind that this doesn't protect your messages from being seen if your device is ever compromised — if someone has access to your phone (or the person you're sending messages to) they can read the messages.
If you want to encrypt your messages on iPhone and Android, here are some tips.