How would I make my first person controller sprint, when I hold left shift?

Take a course on scripting maybe?

well, here is my answer to movement with the ability to sprint. First you need to set up the shift button: Go to >Edit >Project Settings >Input. Once you are there, make the size under the axis text 16 or one more than it was. The default will be Jump, rename it to RightShift. Beside the "positive button" text, type " right shift". Then make a C# script called move. Then copy this script in that script. Hope this helps...

 using UnityEngine;
 using System.Collections;
 
 
 public class move : MonoBehaviour {
     
     //Author :Electric Fountain Studios
     
     //Getting Player
     public GameObject player;
     
     //Declaring Variables
     public float movementspeed = 7.5f;
     public float mouseSensitivity = 5.0f;
     float verticleRotation = 0;
     public float mouseRange = 60.0f;
     public float jumpSpeed = 7.5f;
     public float verticleVelocity = 0;
     float maintainSpeedAtZero = 0.0f;
     float sprint;
         
     //Calling Character Controller    
     CharacterController characterController;
     
     
     void Start () {
         
         //Locking Cursor
         Screen.lockCursor = true;
          characterController = GetComponent<CharacterController>();
     }
     
     
     void Update () {
     
         //Rotation
         
         float rotleftright = Input.GetAxis ("Mouse X")* mouseSensitivity;
         transform.Rotate(0, rotleftright, 0);
         
         verticleRotation -= Input.GetAxis("Mouse Y")* mouseSensitivity;
         verticleRotation = Mathf.Clamp(verticleRotation, -mouseRange, mouseRange);
         Camera.main.transform.localRotation = Quaternion.Euler(verticleRotation, 0, 0);
         
         
         
         //Sprinting
         if(Input.GetButton("RightShift") ) {
             sprint = Mathf.Sqrt(10.0f);
             
         
         }else{
             sprint = sprint - sprint;
         }
             
         
         //Movement    
         float forwardSpeed = Input.GetAxis("Vertical")*movementspeed + sprint;
         
         float sideSpeed = Input.GetAxis("Horizontal")*movementspeed;
         
         
         verticleVelocity += Physics.gravity.y * Time.deltaTime;
         
         if(characterController.isGrounded && Input.GetButtonDown("Jump") ) {
             verticleVelocity = jumpSpeed;
             
         
         }
         
         Vector3 speed = new Vector3 ( sideSpeed, verticleVelocity , forwardSpeed );
         
         speed = transform.rotation * speed;
             
         
         
         characterController.Move( speed * Time.deltaTime );
     }
     
     
 }
 

Keep a temporary variable to act as your final speed, and a constant that is your normal speed, along with another variable to act as a multiplier. A simple example:

 var NaturalSpeed = 10;
 var tempSpeed : float;
 var SpeedMultiplier = 1.2; // for 20% speed increase while sprinting
 
 var moveDirection : Vector3; //this determines which direction we should be moving towards
 
 function Update()
 {
     moveDirection = Vector3.zero; //this rests our move direction to nothing so we wouldn't move unless movement keys were pressed
     tempSpeed = NaturalSpeed; //this rests our speed back to natural speed
 
     //this is just a simple movement check for demonstrating sprint; use your own methods of course
     moveDirection.x += Input.GetAxis("Horizontal");
     moveDirection.z += Input.GetAxis("Vertical");
 
     if(Input.GetKey(KeyCode.LeftShift)) //this checks to see if the player is holding left shift
     {
         tempSpeed *= SpeedMultiplier; //this increases our speed by the multiplier
     }
 
     transform.Translate(moveDirection.normalized * tempSpeed * Time.deltaTime); //this moves our player to the correct direction at the determined speed (sprinted or not), smoothed over time
 }