Need help with rotating around an object

I need help, I want to rotate my camera using my player as an origin when I press e, the problem always seems to be that the camera just rotates and doesn’t use the player as a target, any help would be deeply appreciated.

using UnityEngine;
using System.Collections;
public class CameraController : MonoBehaviour
{
    public GameObject player;  //The offset of the camera to centrate the player in the X axis
    public float offsetX = 24;  //The offset of the camera to centrate the player in the Z axis
    public float offsetZ = -18;  //The maximum distance permited to the camera to be far from the player, its used to     make a smooth movement
    public float maximumDistance = 2;  //The velocity of your player, used to determine que speed of the camera
    public float playerVelocity = 10;
    public float rotatespeed = 2;

    private float movementX;
    private float movementZ;

    void Start()
    {

    }

    // Update is called once per frame
    void Update()
    {
        movementX = ((player.transform.position.x + offsetX - this.transform.position.x)) / maximumDistance;
        movementZ = ((player.transform.position.z + offsetZ - this.transform.position.z)) / maximumDistance;
        this.transform.position += new Vector3((movementX * playerVelocity * Time.deltaTime), 0, (movementZ * playerVelocity * Time.deltaTime));

        if (Input.GetKey("e"))
        {

        }

    }
}

Check out this tutorial