MonoBehaviour.Start()

Description

Start is called on the frame when a script is enabled just before any of the Update methods are called the first time.

Start is called exactly once in the lifetime of the script and always after MonoBehaviour.Awake. Start might not be called on the same frame as Awake if the script is not enabled at initialization time.

Start is not called on any object until Awake has been called on every object in the scene. In cases where object A's initialization code relies on object B already being initialized, you can initialize B in B's Awake and initialize A in A's Start.

If you instantiate objects at runtime from MonoBehaviour.Update, Awake and Start are called on the instantiated objects before the end of the current frame.

Start can be defined as a coroutine.

// Initializes the target variable.
// target is private and thus not editable in the Inspector

// The ExampleClass starts with Awake.  The GameObject class has activeSelf
// set to false.  When activeSelf is set to true the Start() and Update()
// functions will be called causing the ExampleClass to run.
// Note that ExampleClass (Script) in the Inspector is turned off.  It
// needs to be ticked to make script call Start.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    private float update;

    void Awake()
    {
        Debug.Log("Awake");
        update = 0.0f;
    }

    IEnumerator Start()
    {
        Debug.Log("Start1");
        yield return new WaitForSeconds(2.5f);
        Debug.Log("Start2");
    }

    void Update()
    {
        update += Time.deltaTime;
        if (update > 1.0f)
        {
            update = 0.0f;
            Debug.Log("Update");
        }
    }
}