On your iPhone, go to Settings > Messages.
Tap Text Message Forwarding.
Tip: If you don’t see Text Message Forwarding, make sure you’re signed in to iMessage using the same Apple ID on both your iPhone and your Mac.
Enable your Mac in the list of devices.
On your Mac, open the Messages app iPod touch. For a device that doesn’t use the latest iOS version, get the user guide from the Apple Support article: Use Messages with your Mac