3rd person camera-based player movement

I want to make the player to move on the A/D axis when the camera rotates like in most 3rd person games.

I have managed to make the camera follow the player and rotate with him but it seems that whenever the player rotates, its W/S axis does not rotate with him so he moves towards the same direction but rotated.

I believe I am approaching this wrong so I won't provide any of my code. Instead it would be really helpful if someone provided a complete answer on how to move the player with the camera left and right.

I have read previous questions connected to this matter but I am afraid I didn't make any progress.

Thanks in advance.