problema no setcookie()

por favor me ajudemmmmmmmmta dando o sgeuinte erro quando eu fasso isso:} else { $user = mysql_result($query,0,id); setcookie("user", "$user"); setcookie("acesso", "sim");}erro: Warning: Cannot add header information - headers already sent by (output started at c:\apache\htdocs\tubacity\quiz\index.php:13) in c:\apache\htdocs\tubacity\adm\user\gincana.php on line 20Warning: Cannot add header information - headers already sent by (output started at c:\apache\htdocs\tubacity\quiz\index.php:13) in c:\apache\htdocs\tubacity\adm\user\gincana.php on line 21pqq???????????????