Form question

[BlackRiver]

So I have a simple login form.I can enter username and password,submit the data to my script.The script checks if username and password are correct.

The question:
How do I start a session only if the data submitted is correct?

I don't want to have:

<?php
session_start(); 
$_SESSION['username'] = $_POST['username']; 
$_SESSION['password'] = $_POST['password'];
?>

at the very top of my login.php,because even if the data is not correct,the form is still submited,so if I use:

<?php
session_start();  
if(isset($_SESSION['username'])) {
    if (isset($_SESSION['password']))
{

the user would still be logged in.