Either enable OK Google or open the Google app and tap the microphone. Say 'send a text', it will prompt you through the rest. When you get used to it, it can take longer commands such as 'send a text message to Bob Smith mobile'
Welcome to Android Central! If you're using the Gboard keyboard app, open its settings and make sure Voice Input Key is switched on. You should see the microphone now at the upper right of the keyboard.