How to write a apex trigger.

SetUp - > Obejct Name -> Trigger -> New

trigger triggerName on ObjectName (trigger_events) {
      //code_block
}

Example : Update all opportunities Stage Field. 

Trigger myTrigger on Account(after update) 
    { 
       Set<Id> setAccId = new Set<Id>();       
       for(Account a: Trigger.old) {
            if(a.CloseOpportunities__c) 
                setAccId.add(a.Id);
        }   
            
       List<Schema.Opportunity> lstOpp = [select Id, StageName from Opportunity
                                          where AccountId in : setAccId ] ;        
       for(Integer i = 0; i < lstOpp.size(); i++){ 
          lstOpp[i].StageName = 'Closed-Won'; 
        } 
          
       if(lstOpp.size() > 0) 
           update lstOpp;   
   }