trigger ClosedOpportunityTrigger on Opportunity (before insert,before update) {
    List<task> tsklst=new List<task>();
   List<Opportunity> opplst=new List<Opportunity>([select id,StageName from opportunity where id in:Trigger.New]);
    //List<Opportunity> opplst=new List<Opportunity>();
    if(Trigger.isInsert || Trigger.isUpdate)
    {
       if(opplst.StageName =='Closed Won')
        {
            for(Opportunity opp:Trigger.new){
                
                system.debug('@@@@@@@' +opp);
         Task t=New Task();
             t.WhatId = opp.id;
            t.Subject = 'Follow Up Test Task';
            tsklst.add(t);
            } 
        }
        insert tsklst;
        
    }
}