Sessions em PHP

Tipo quando eu executo a index.php ele retorna este erro:
Warning: session_start(): Cannot send session cookie - headers already sent by (output started at C:\Apache2\htdocs\index.php:9) in C:\Apache2\htdocs\index.php on line 10

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at C:\Apache2\htdocs\index.php:9) in C:\Apache2\htdocs\index.php on line 10

As linhas 9 e 10:

<?php
session_start();
session_register("numero");

if (!isset($numero)) { $numero = 1; }
?>

Tem algo de errado ?
Sou noob no assunto...