How do I aim for the gun follow the mouse?

I’m doing a 3D-game, Side-Scrolling, and I need the shot to be done in the direction the mouse is, but I have no idea how to do it.

Can someone help me?

google Raycasting, furthermore google basic tutorials that cover aiming and side scrollers, most contain scripts. if you have no scripting experience making a sidescroller would be difficult.