Adding stamina to the first person controller

I want to add a stamina system to the first person controller, but idk where to add it into
it’s this script

using System;
using UnityEngine;
using UnityStandardAssets.CrossPlatformInput;
using UnityStandardAssets.Utility;
using Random = UnityEngine.Random;

namespace UnityStandardAssets.Characters.FirstPerson

{
[RequireComponent(typeof (CharacterController))]
[RequireComponent(typeof (AudioSource))]
public class FirstPersonController : MonoBehaviour
{
[SerializeField] private bool m_IsWalking;
[SerializeField] private float m_WalkSpeed;
[SerializeField] private float m_RunSpeed;
[SerializeField] [Range(0f, 1f)] private float m_RunstepLenghten;
[SerializeField] private float m_JumpSpeed;
[SerializeField] private float m_StickToGroundForce;
[SerializeField] private float m_GravityMultiplier;
[SerializeField] private MouseLook m_MouseLook;
[SerializeField] private bool m_UseFovKick;
[SerializeField] private FOVKick m_FovKick = new FOVKick();
[SerializeField] private bool m_UseHeadBob;
[SerializeField] private CurveControlledBob m_HeadBob = new CurveControlledBob();
[SerializeField] private LerpControlledBob m_JumpBob = new LerpControlledBob();
[SerializeField] private float m_StepInterval;
[SerializeField] private AudioClip m_FootstepSounds; // an array of footstep sounds that will be randomly selected from.
[SerializeField] private AudioClip m_JumpSound; // the sound played when character leaves the ground.
[SerializeField] private AudioClip m_LandSound; // the sound played when character touches back on ground.

    private Camera m_Camera;
    private bool m_Jump;
    private float m_YRotation;
    private Vector2 m_Input;
    private Vector3 m_MoveDir = Vector3.zero;
    private CharacterController m_CharacterController;
    private CollisionFlags m_CollisionFlags;
    private bool m_PreviouslyGrounded;
    private Vector3 m_OriginalCameraPosition;
    private float m_StepCycle;
    private float m_NextStep;
    private bool m_Jumping;
    private AudioSource m_AudioSource;

    // Use this for initialization
    private void Start()
    {
        m_CharacterController = GetComponent<CharacterController>();
        m_Camera = Camera.main;
        m_OriginalCameraPosition = m_Camera.transform.localPosition;
        m_FovKick.Setup(m_Camera);
        m_HeadBob.Setup(m_Camera, m_StepInterval);
        m_StepCycle = 0f;
        m_NextStep = m_StepCycle/2f;
        m_Jumping = false;
        m_AudioSource = GetComponent<AudioSource>();
		m_MouseLook.Init(transform , m_Camera.transform);
    }

    // Update is called once per frame
    private void Update()
    {
        RotateView();
        // the jump state needs to read here to make sure it is not missed
        if (!m_Jump)
        {
            m_Jump = CrossPlatformInputManager.GetButtonDown("Jump");
        }

        if (!m_PreviouslyGrounded && m_CharacterController.isGrounded)
        {
            StartCoroutine(m_JumpBob.DoBobCycle());
            PlayLandingSound();
            m_MoveDir.y = 0f;
            m_Jumping = false;
        }
        if (!m_CharacterController.isGrounded && !m_Jumping && m_PreviouslyGrounded)
        {
            m_MoveDir.y = 0f;
        }

        m_PreviouslyGrounded = m_CharacterController.isGrounded;
    }

    private void PlayLandingSound()
    {
        m_AudioSource.clip = m_LandSound;
        m_AudioSource.Play();
        m_NextStep = m_StepCycle + .5f;
    }

    private void FixedUpdate()
    {
        float speed;
        GetInput(out speed);
        // always move along the camera forward as it is the direction that it being aimed at
        Vector3 desiredMove = transform.forward*m_Input.y + transform.right*m_Input.x;

        // get a normal for the surface that is being touched to move along it
        RaycastHit hitInfo;
        Physics.SphereCast(transform.position, m_CharacterController.radius, Vector3.down, out hitInfo,
                           m_CharacterController.height/2f);
        desiredMove = Vector3.ProjectOnPlane(desiredMove, hitInfo.normal).normalized;

        m_MoveDir.x = desiredMove.x*speed;
        m_MoveDir.z = desiredMove.z*speed;

        if (m_CharacterController.isGrounded)
        {
            m_MoveDir.y = -m_StickToGroundForce;

            if (m_Jump)
            {
                m_MoveDir.y = m_JumpSpeed;
                PlayJumpSound();
                m_Jump = false;
                m_Jumping = true;
            }
        }
        else
        {
            m_MoveDir += Physics.gravity*m_GravityMultiplier*Time.fixedDeltaTime;
        }
        m_CollisionFlags = m_CharacterController.Move(m_MoveDir*Time.fixedDeltaTime);

        ProgressStepCycle(speed);
        UpdateCameraPosition(speed);
    }

    private void PlayJumpSound()
    {
        m_AudioSource.clip = m_JumpSound;
        m_AudioSource.Play();
    }

    private void ProgressStepCycle(float speed)
    {
        if (m_CharacterController.velocity.sqrMagnitude > 0 && (m_Input.x != 0 || m_Input.y != 0))
        {
            m_StepCycle += (m_CharacterController.velocity.magnitude + (speed*(m_IsWalking ? 1f : m_RunstepLenghten)))*
                         Time.fixedDeltaTime;
        }

        if (!(m_StepCycle > m_NextStep))
        {
            return;
        }

        m_NextStep = m_StepCycle + m_StepInterval;

        PlayFootStepAudio();
    }

    private void PlayFootStepAudio()
    {
        if (!m_CharacterController.isGrounded)
        {
            return;
        }
        // pick & play a random footstep sound from the array,
        // excluding sound at index 0
        int n = Random.Range(1, m_FootstepSounds.Length);
        m_AudioSource.clip = m_FootstepSounds[n];
        m_AudioSource.PlayOneShot(m_AudioSource.clip);
        // move picked sound to index 0 so it's not picked next time
        m_FootstepSounds[n] = m_FootstepSounds[0];
        m_FootstepSounds[0] = m_AudioSource.clip;
    }

    private void UpdateCameraPosition(float speed)
    {
        Vector3 newCameraPosition;
        if (!m_UseHeadBob)
        {
            return;
        }
        if (m_CharacterController.velocity.magnitude > 0 && m_CharacterController.isGrounded)
        {
            m_Camera.transform.localPosition =
                m_HeadBob.DoHeadBob(m_CharacterController.velocity.magnitude +
                                  (speed*(m_IsWalking ? 1f : m_RunstepLenghten)));
            newCameraPosition = m_Camera.transform.localPosition;
            newCameraPosition.y = m_Camera.transform.localPosition.y - m_JumpBob.Offset();
        }
        else
        {
            newCameraPosition = m_Camera.transform.localPosition;
            newCameraPosition.y = m_OriginalCameraPosition.y - m_JumpBob.Offset();
        }
        m_Camera.transform.localPosition = newCameraPosition;
    }

    private void GetInput(out float speed)
    {
        // Read input
        float horizontal = CrossPlatformInputManager.GetAxis("Horizontal");
        float vertical = CrossPlatformInputManager.GetAxis("Vertical");

        bool waswalking = m_IsWalking;

#if !MOBILE_INPUT
// On standalone builds, walk/run speed is modified by a key press.
// keep track of whether or not the character is walking or running
m_IsWalking = !Input.GetKey(KeyCode.LeftShift);
#endif
// set the desired speed to be walking or running
speed = m_IsWalking ? m_WalkSpeed : m_RunSpeed;
m_Input = new Vector2(horizontal, vertical);

        // normalize input if it exceeds 1 in combined length:
        if (m_Input.sqrMagnitude > 1)
        {
            m_Input.Normalize();
        }

        // handle speed change to give an fov kick
        // only if the player is going to a run, is running and the fovkick is to be used
        if (m_IsWalking != waswalking && m_UseFovKick && m_CharacterController.velocity.sqrMagnitude > 0)
        {
            StopAllCoroutines();
            StartCoroutine(!m_IsWalking ? m_FovKick.FOVKickUp() : m_FovKick.FOVKickDown());
        }
    }

    private void RotateView()
    {
        m_MouseLook.LookRotation (transform, m_Camera.transform);
    }

    private void OnControllerColliderHit(ControllerColliderHit hit)
    {
        Rigidbody body = hit.collider.attachedRigidbody;
        //dont move the rigidbody if the character is on top of it
        if (m_CollisionFlags == CollisionFlags.Below)
        {
            return;
        }

        if (body == null || body.isKinematic)
        {
            return;
        }
        body.AddForceAtPosition(m_CharacterController.velocity*0.1f, hit.point, ForceMode.Impulse);
    }
}

}

Where can i had the stamina system?

I read your question and got intrested in the same thing. so i changed the script to handle stamina when the first person is running. I need to say that i am very new to unity and game development, but here is how i did this.

Below private AudioSource m_AudioSource; witch is on line 43 you add

public int stamina;
private int staminaStart;

And inside private void Start() { you add

stamina = 500;staminaStart = stamina-1;

stamina variable is the max amout of stamina your first person vill have and staminaStart is just so we remeber how much it was set

Then inside private void FixedUpdate() { below desiredMove=Vector3.ProjectOnPlane(desiredMove, hitInfo.normal).normalized;
you add

if (Input.GetKey(KeyCode.LeftShift) && m_MoveDir.x != desiredMove.x * speed && m_MoveDir.z != desiredMove.z * speed) {
                if (stamina <= 0) {
                    stamina = 0;
                } else { 
                    stamina = stamina -1;
                }
            } else if (!Input.GetKey(KeyCode.LeftShift))
            {   
                if (stamina <= staminaStart) {
                    stamina = stamina + 1;
                }
            }

Here we check if you are holding shift and that you are not standing still, and if you are running we reduce the stamina and if the player is walking or standing still we add new stamina. and last we go down to private void GetInput(out float speed) { and below #endif
we change speed = m_IsWalking ? m_WalkSpeed : m_RunSpeed; to

if (stamina == 0)
            {
                speed = m_WalkSpeed; 
            } else
            {
                speed = m_IsWalking ? m_WalkSpeed : m_RunSpeed;
            }

This stops the player to be able to run if the stamina is at 0.

The reason i use public for stamina variable was just to se if the changes worked. you can add stamina to a slider or something to se the stamina.

As i said im new to this, but this seems to work. There is probably alot of better way to do this. but hope this helps