public float speed = 6.0F;
public float jumpSpeed = 8.0F;
public float gravity = 20.0F;
private Vector3 moveDirection = Vector3.zero;
void Update() {
    CharacterController controller = GetComponent<CharacterController>();
    if (controller.isGrounded) {
        moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
        moveDirection = transform.TransformDirection(moveDirection);
        moveDirection *= speed;
        if (Input.GetButton("Jump"))
            moveDirection.y = jumpSpeed;

    }
    moveDirection.y -= gravity * Time.deltaTime;
    controller.Move(moveDirection * Time.deltaTime);
}