I am usinf after insert Trigger.

trigger import_into_stg on STG_AR_INVOICE__c (after insert) {

// Create new Customer
 List <Account> toBeInserted = new List<Account>();

  // Instantiate a list of Accounts to update
  List< Account> aList = new List<Account>();
  List<Account> allAcc= new List<Account>();
   
  util t=new util(); 
  
  // Instantiate a list of STG_AR_INVOICE to populate Customer Key  
  List< STG_AR_INVOICE__C > stgList = new List< STG_AR_INVOICE__C >();
  List< STG_AR_INVOICE__C > updatestg = new List< STG_AR_INVOICE__C >();
    
  Boolean updateFlage;
    
  for ( STG_AR_INVOICE__c stg : Trigger.new ){    
           
           Account[] a=[SELECT NAME,COMPANY_CODE__c,BU_CODE__C,SOURCE_CUSTOMER_ID__c FROM ACCOUNT 
                 WHERE BU_CODE__c =:stg.BU_CODE__C AND NAME =:stg.CUSTOMER_NAME__c AND COMPANY_CODE__c =:stg.COMPANY_CODE__c];
               
          Account b;                   
          IF(a.size()>=1){  
              for(integer i=0;i<a.size();i++){
                 b=a[i];
              }                   
              b.NAME =stg.CUSTOMER_NAME__c;
              b.Credit_Limit__c=stg.Credit_Limit__c; 
              b.Credit_Available__c=stg.Credit_Available__c;
              b.Net_Credit_Available__c=stg.Net_Credit_Available__c;
              b.OutStanding_Orders__c=stg.OutStanding_Orders__c;
              b.Out_Standing_Net_AR__c=stg.OUT_STANDING_NET_AR__c;         
              aList.add( b );
              
              // Update the Account
              if(aList.size()>=1){
              update aList;
              updateFlage = TRUE;
              }         
            
           //Back in Staging table and populate record key       
         if(updateFlage){   
          STG_AR_INVOICE__C [] stgexist=[SELECT ID,NAME,COMPANY_CODE__c,BU_CODE__C,SOURCE_CUSTOMER_ID__c,CUSTOMER_KEY__C FROM STG_AR_INVOICE__C 
                 WHERE BU_CODE__c =:stg.BU_CODE__C AND SOURCE_CUSTOMER_ID__c =:stg.SOURCE_CUSTOMER_ID__c AND COMPANY_CODE__c =:stg.COMPANY_CODE__c];
          STG_AR_INVOICE__C stgupdate;
          if(stgexist.size()>0){
            for(integer i=0;i<stgexist.size();i++){
             stgupdate=stgexist[i];
          }    
            stgupdate.CUSTOMER_KEY__C=b.Id;
            stgList.add(stgupdate); 
            
            
          }  
         }    
          }else{
              updateFlage = false;               
              // loop through trigger records
               //System.debug('In Trigger Create Customer');              
             
               Account CreateAccnt = new Account();
               CreateAccnt.Name =stg.CUSTOMER_NAME__c;
               CreateAccnt.COMPANY_CODE__c=stg.COMPANY_CODE__c;
               CreateAccnt.BU_CODE__c=stg.BU_CODE__c;
               
               CreateAccnt.Credit_Limit__c=stg.Credit_Limit__c;
               CreateAccnt.Credit_Available__c=stg.Credit_Available__c;
               CreateAccnt.Net_Credit_Available__c=stg.Net_Credit_Available__c;
               CreateAccnt.OutStanding_Orders__c=stg.OutStanding_Orders__c;
               CreateAccnt.Out_Standing_Net_AR__c=stg.OUT_STANDING_NET_AR__c;
                      
               toBeInserted.add(CreateAccnt);

             
             
             // Update Staging table

             String accId=t.getAccountId(stg.BU_CODE__C,stg.Company_Code__c,stg.Customer_Name__c);
             
             if(accId !=null){
                 STG_AR_INVOICE__C [] updstg=[SELECT ID,NAME,COMPANY_CODE__c,BU_CODE__C,SOURCE_CUSTOMER_ID__c,CUSTOMER_KEY__C FROM STG_AR_INVOICE__C 
                 WHERE BU_CODE__c =:stg.BU_CODE__C AND SOURCE_CUSTOMER_ID__c =:stg.SOURCE_CUSTOMER_ID__c AND COMPANY_CODE__c =:stg.COMPANY_CODE__c];
              STG_AR_INVOICE__C stgupdate;
             if(updstg.size()>0){
                for(integer i=0;i<updstg.size();i++){
                 stgupdate=updstg[i];
             }    
                stgupdate.CUSTOMER_KEY__C=b.Id;
                updatestg.add(stgupdate);           
               
              }  
            }           
          }               
        
        
  }    
  
  if(stgList.size()>0){
              update stgList;
           }
        if(toBeInserted.size()>0){
                 insert toBeInserted;
                 updateFlage =true;
        }
        
        if(updatestg.size()>0 && updateFlage){
                 update updatestg;
        }  
        updateFlage =false;
}

Apex Class 

public class util{

public List<Account> copyCustomersFromERP(String bu_code,String company_code,String customer_name,Double credit_limit){

 // Check First Customer exist or not.
 //private isCustomerExist=false; 
  List <Account> toBeInserted = new list<Account>(); 
  /*
  String bu_code='100';
  String company_code='FMA';
  String source_customer_id='DELL';
  */
  /*
  Integer count=[SELECT Count() FROM Account act 
  Where act.BU_CODE__c=: bu_code AND act.COMPANY_CODE__c=:company_code AND act.NAME =:customer_name];
  */
   
   
   //IF(count <= 0){
     // loop through trigger records
     //for (Integer i=0; i<Trigger.new.size(); i++){       
               Account CreateAccnt = new Account();
               CreateAccnt.Name = customer_name;
               CreateAccnt.COMPANY_CODE__c= company_code;
               CreateAccnt.BU_CODE__c = bu_code;
               CreateAccnt.Credit_Limit__c=credit_limit;       
               toBeInserted.add(CreateAccnt);
     // }

      //}  
      return toBeInserted;
  }
  
  
}

 I am new in Apex trigger . 

When I am Inserting bulk data then it is showing Exception

System.Exception: Too many SOQL queries: 21 

Please any Body Suggest me how can I do. Please correct my code.

Please also describe me how "after trigger" works. I have confusion that whether it work row by row or after complete all insertion.

Thanks in Advance !!!