Rotate object to lookAt point and be perpendicular to the ground

My game is a top down game, and to move the player object it just moves towards the mouse point. Right now I have it setup so that the object orients itself to face the point where the user clicked and the object moves to that point. However what I am having an issue with is having the player object face the mouse point and have the player object to correctly stay perpendicular to the ground. Any ideas how to do this?

use this method:

transform.LookAt();