Animator Null refrence exception

I have a tiger model which has two animation walking and running i have made the transition and i want tiger to run when i press key i declared the animator component in strt method and was trying to set run variable in animator to 1 by code but it gave me this errror

NullReferenceException: Object reference not set to an instance of an object
NewBehaviourScript.Start () (at Assets/NewBehaviourScript.cs:15)

the code is below

void Start () {


Are you sure you are actually getting the animator. Try Debug.Log(anim) after you get component. It appears that when you call GetComponent it is returning null.