Added start countdown

Small UI changes
Fixed initialization order for the GameManager

Assets/Scripts/Countdown.cs

@@ -0,0 +1,73 @@
+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+using UnityEngine.UI;
+
+public class Countdown : MonoBehaviour
+{
+
+    public float seconds = 3;
+    public bool lockControls = true;
+    public AudioClip tickSound;
+    public AudioClip startSound;
+    public AudioSource audioSource;
+    public GameObject[] enableOnZero;
+    public GameObject[] disableOnZero;
+    public string finalText = "Go!";
+    public float finalTextScreentime = 0.5f;    //How long should we display the final text
+    public Text uiText;
+
+    public float remaining;
+
+    // Start is called before the first frame update
+    void Start()
+    {
+        if (this.lockControls)
+        {
+            GameObject.FindGameObjectWithTag("GameManager").GetComponent<GameManager>().LockControls(this.seconds);
+        }
+
+        this.remaining = this.seconds;
+    }
+
+    // Update is called once per frame
+    void Update()
+    {
+        int second = (int)Mathf.Ceil(this.remaining);
+        this.UpdateText(second);
+        
+
+        if(this.remaining <= 0)
+        {
+            this.ProcessGameObjects(this.enableOnZero, true);
+            this.ProcessGameObjects(this.disableOnZero, false);
+        }
+
+        if(this.remaining <= -finalTextScreentime)
+        {
+            this.uiText.gameObject.SetActive(false);
+            this.enabled = false;
+        }
+
+        this.remaining -= Time.deltaTime;
+    }
+
+    private void UpdateText(int second)
+    {
+        string newText = this.remaining > 0 ? second.ToString() : this.finalText;
+
+        if(newText != this.uiText.text)
+        {
+            this.uiText.text = newText;
+            this.audioSource.PlayOneShot(newText == this.finalText ? this.startSound : this.tickSound);
+        }
+    }
+
+    private void ProcessGameObjects(GameObject[] objectList, bool enable)
+    {
+        for (int i = 0; i < objectList.Length; i++)
+        {
+            objectList[i].SetActive(enable);
+        }
+    }
+}