Trouble with Script for in-game Cursor

Scripting
1

Hello, I am relatively new to Unity. I am trying to create a script to attach to a gameobject that will become a cursor for a game I am making. Here is my current script.

using UnityEngine;
using System.Collections;

public class Aim : MonoBehaviour {

Vector3 position = Camera.main.ScreenToWorldPoint(Input.mousePosition);
position.z = 0;

void Start(){

}

void Update(){
transform.position = position;
}
}

Currently it does not work. Is there a way to fix this?

2 
using UnityEngine;

public class Aim : MonoBehaviour {

    void Update()
    {
       
         //The last bit is important. You need to offset the position from the cameras origin, in some way,
         //otherwise the resulting worldPoint will always be 0,0,0
        Vector3 screenPoint = Input.mousePosition + Vector3.forward * 10f;
        Vector3 worldPoint = Camera.main.ScreenToWorldPoint(screenPoint);
       
        //Notice that I'm not caching the variable or declaring it in the class. You COULD do this,
        //but since you need to recalculate it every frame anyway, I see no point.
        //The main thing that was wrong with your original script is that you only calculated
        //the screen point once, and you did it IN the declaration, which I would have expected to
        //throw a thread error.
        transform.position = worldPoint;
   
    }

}
3

Also, please use code tags, in the future.