What do you insert in the "" area in this timer? (00:00)

Ok first of all i think you want to make a timer with this format: minutes:seconds right? In your program there are various errors the most obvious seems to me that count is never defined. I wrote a short script here to help you.

 using UnityEngine;
 using System.Collections;
 //Needed to access Text component
 using UnityEngine.UI;
 
 public class Timer : MonoBehaviour 
 {
     //you need a Text component attached to the gameobject
     Text timerText;
     public float myTimer = 90.0f;
 
     void Start()
     {
         //Initialize the text component
         timerText = GetComponent<Text> ();
     }
 
     void Update ()
     {
         SetTimerText();
         //Only decrease the timer when it is greater than 0 otherwise make sure that the timer is is not going to the negative part
         if (myTimer > 0.0f)
         {
             myTimer -= Time.deltaTime;
         } else
             myTimer = 0.0f;
     }
     void SetTimerText()
     {
         timerText.text = GetMinutes ().ToString ("00") + ":" + GetSeconds ().ToString ("00");;
     }
 
     float GetMinutes()
     {
         //return the greater integer when dividing -> 5/3 = 2
         return Mathf.Floor (myTimer / 60);
     }
 
     float GetSeconds()
     {
         //Modulus(remainder) of timer -> the remainder when dividing timer with 60 --> 125/60 = 2 + remainder 5
         return myTimer % 60;
     }
 }

I hope it helps you -Jorge