trigger fieldUpdate on Opportunity (after update) {

   List<Task> tasks = new List<Task>();
   List<Opportunity> Opps = Trigger.new;
   Map <Id, String> oppsOwner = new Map <Id, String> ();
       // Go through every opportunity in the trigger
    for (Opportunity opp : trigger.new)
    {
        // Check if the StageName has been changed
        if (opp.StageName != trigger.oldMap.get(opp.Id).StageName)
        {
            // get the owner ID's that have been affected
            oppsOwner.put(opp.ownerId, null);
        }
    }

    // Map the owner ID to it's email address
    for (User owner : [SELECT Id, Email FROM User WHERE Id = :oppsOwner.keySet()])
    {
        oppsOwner.put(owner.Id, owner.Email);
    }
    
    
   for (Opportunity Opp : Opps)
   {
     if(opp.StageName == 'Closed Won' && trigger.oldMap.get(opp.Id).StageName != opp.StageName) {
     
     Task tsk = new Task(whatID = Opp.ID, Ownerid = Opp.OwnerId);
     tasks.add(tsk); 
    }  
   } 
   insert tasks;
    
 
    List <Messaging.SingleEMailMessage> emails = new List <Messaging.SingleEMailMessage> ();

    // Go again through every opportunity in the trigger
    for (Opportunity opp : trigger.new)
    {  
        // Only work with opportunities that have owners mapped to their email addresses (only those ones have their status changed)
        if (oppsOwner.get(opp.OwnerId) != null)
        {
            // Create an email message and add it to the list
            Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
            List <String> toAddresses = new List <String> {oppsOwner.get(opp.ownerId)};
            // set it in meassage as
            mail.setToAddresses(toAddresses);
            mail.setSubject('Automated Email : Opportunity StageName Updated');
            String body = 'The status has been changed on the opportunity record with StageName ' + opp.StageName;
            mail.setPlainTextBody(body);
            emails.add(mail);
        }
    }
    Messaging.sendEmail(emails);
       
  }