Quick Scripting Question!?

How do I make it so that after clicking somewhere with your mouse (with a teleport script done which makes you teleport to where you click) and leave a decal image at your previous spot?

Assuming you're only wondering about the leaving of a decal, I would get the position of the character's torso before it's teleported, create a new Part(put the part at old position of torso, making customizations as you want them) make it transparent if you wish, and put the desired decal on the top of it.