Display the Average in an SQL query

I want to perform a search on a query so far I have only been able to display the total value of the logs.  the idea is to show the average spending. Then it would be the (total / quantity) but I could not mount the logic: what I have so far that presents the count of all the records

with AdoMEdOs do
begin
   Close;
   Sql.clear;
   AdoMEdOs.SQL.Add('select valor_total from ordem_servico');
   open;
   RecordCount;
   label5.Caption := IntToStr(AdoMEdOs.RecordCount) ;
   AdoMEdOs.Close;
end;