How to access a variable on another script

When I’m trying to access a variable and methos on another script, I’m getting an error:

MoveSphere.cs:

public class MoveSphere : MonoBehaviour
{
	void Start()
	{
        // NullReferenceException: Object reference not set
        // to an instance of an object MoveSphere.Start ()
	    GetComponent<TargetScript> ().myVar = 5;
	    GetComponent<TargetScript> ().ShowVar ();
	}
}

TargetScript.cs

public class TargetScript : MonoBehaviour
{
    public int myVar;

    public void ShowVar()
    {
        Debug.Log (myVar);
    }
}

The only error i can think about is you get MissingComponentException because your TargetScript is not on the same object as your MoveSphere.

Either fix this, or find the gameobject that has got a TargetScript component.
You can use GameObject.Find();

or

you can have a public GameObject member in your MoveSphere script and assign it in the inspector with the object that has TartScript component