How can I turn mic on to enable text messaging

If you're using iOS 9 or 10 and a reasonably current device, to dictate text, tap the mic bottom just to the left of the space bar on the keypad in an iMessage window (top pic). To dictate an audio message, hold down the mic button on the left side of the text entry slot (bottom pic).

Incidentally, please let everyone know what version of iOS you're using and which iOS device you have when asking a question.