Make Character aim to the cursor point

I'm making a Third Person Shooter game. My problem is how to make the character aim exactly to the cursor (crosshair) and shoot. I found some script that make the character rotate to the camera but this doesn't look realistic. I know that Unity has a system call IK, is Unity Free has it too? So how can I make my character's aiming looks more realistic, I mean when we rotate the mouse, he rotate the upper part of body to aim then when we rotate to the back, he rotate his leg ?