Error BCE0018

If you check my profile I get error after error in scripts and when researching this script I found that it is cause monodevelop doesn't recognize the "AnimateTexture" as a class (sorry I'm new) So can you tell me what code and where I should put the code that tells unity how to recognize the class and also what coding language I should begin learning? Thank you guys so much

 #pragma strict
 
 var AT : AnimateTexture; //Store something
 
 function Start () {
     AT = gameObject.GetComponent(AnimateTexture); //Store AnimateTexture Script
 }
 
 function Update () {
     if(Input.GetKey("a")){ //Player moves left
         AT.rowNumber = 1; //Change to running animation
     } else if(Input.GetKey("d")){ //Player moves right
         AT.rowNumber = 1; //Change to running animation
     } else { //Player is not moving
         AT.rowNumber = 0; //Change to idle animation!
     }
     
 }