Movement direction and camera.

Hi. How can I make sure that the direction of motion is equal to the direction in which I'm looking at the camera?

Code (csharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class PlayerMovement : MonoBehaviour {
5.  
6.     public Transform camera;
7.     public float speed;
8.     public float jumpHeight;
9.     public float gravity;
10.  
11.     private CharacterController controller;
12.     private Vector3 moveDirection = Vector3.zero;
13.  
14.    
15.     void Start()
16.     {
17.         controller = gameObject.GetComponent<CharacterController>();
18.     }
19.  
20.     void Update()
21.     {
22.         if(controller.isGrounded)
23.         {
24.             moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
25.             moveDirection = transform.TransformDirection(moveDirection);
26.             moveDirection *= speed;
27.  
28.             if(Input.GetButtonDown("Jump"))
29.             {
30.                 moveDirection.y = jumpHeight;
31.             }
32.         }
33.  
34.         moveDirection.y -= gravity * Time.deltaTime;
35.  
36.         controller.Move(moveDirection * Time.deltaTime);
37.     }
38. }
39.