How can I make a object rotate along the z-axis as it orbits around a point?

I am still a newbie to this, so please excuse my sloppy probably highly inefficient code. I am trying to get a object, say a square, to rotate along the z-axis as it physically orbits around another object so that one side always faces the object in the middle, It also has to be controlled by the mouse position. I have done all of the math to get it to orbit using the mouse position, I just need to get it to rotate.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Shield : MonoBehaviour {
	Vector2 Mouse;
	public GameObject body;
	public float distance;
	public float radius = 2;
	// Use this for initialization
	void Start () {
	// Update is called once per frame
	void Update () {
		Mouse = new Vector2 ((Input.mousePosition.x / Screen.width * 13.333333333f) - (10f/1.5f), (Input.mousePosition.y / Screen.height * 10) - 5f);
		distance = Mathf.Sqrt((Mouse.x - body.transform.position.x) * (Mouse.x - body.transform.position.x) + (Mouse.y - body.transform.position.y) * (Mouse.y - body.transform.position.y));
		Mouse = new Vector2 ((Mouse.x / distance) * radius, (Mouse.y / distance) * radius);
		this.transform.position = Mouse;

Say you want to make a square orbit. You want to make the square game object a child of another game object and move the square away from the parent game object. When you rotate the parent game object you will see the square orbiting around the parent game object. This way you can make the square always face the parent object too.
I did not understand what you want to with the mouse position. If you explain that I could help you with that too!