turn on text message alert

Go to Settings/Notifications and turn on Allow Notifications, then chose options for types of alerts.

By default this should send a notification (and sound if you enabled that in the above settings), however, individual message threads can be silenced. In the message list if you slide a message from right to left you will see a bell icon that means the thread has been silenced, tap on the bell to unsilence it and the bell will show with a line through it (meaning “tap to silence”)

If you wear an Watch alerts will only sound on the watch and not the iPhone, unless the iPhone is unlocked.

to save going through every possible setting, read this first Stop, mute, and change message notifications on iPhone - Apple Support