Melee Raycast

Hello everyone. sorry if i format this wrong or something I am trying to make a first person fighting game and I am having problems with hitting and attacking movements. I have tried using the colliders and a target script which lowers a text number the HP on detection. The problem I was having was it caused mutiple detection's and lower'd the HP too much in one punch. I am trying to now use raycast as a means of detection. This is what I am trying to use for the ray, and getting a null reference error on line 18 for some reason. If someone could help with a script that would activate the ray on mouse click to the cursors location, for targeting purposes like hitting head or legs. Also if there is a way that the ray could be followed up visually so it looks like a punching motion.

I am still new to all this and I am trying to teach myself as I go, so if you could also try and explain a little as to why there is a way that works that would be very helpful so hopefully I can fix problems like this on my own in the future.

Thanks

tldr: trying to get a ray that detects hits on mouse click. like a punching movement.

 void Start()
 {
     maincam = GetComponentInParent<Camera>();
 }

 void Update()
 {
     // Create a vector at the center of our camera's viewport
     Vector3 lineOrigin = maincam.ViewportToWorldPoint(new Vector3(0.5f, 0.5f, 0.0f));

     // Draw a line in the Scene View  from the point lineOrigin in the direction of fpsCam.transform.forward , using the color green
     Debug.DrawRay(lineOrigin, maincam.transform.forward , Color.green);
 }

}