Help with email signup (single textbox emailed to myself)

On each page of my website I have a form, I want this to be the users email sent to my email. So far I have this but no joy on the send part. What am I doing wrong.

<form action="" method="post">
<?php
$sent = $_POST['newsletter'];
if (!$sent) {
echo '<input name="newsletter-email" type="text" id="wo" />
<input type="image" name="newsletter" id="wogo" src="img/wogo.png" />';
}
elseif ($sent) {
echo 'Sent';
}
?>

</form>

Try this,
<form action="" method="post">
<?php
$sent = $_POST['newsletter'];
mail("you@yoursite.com","Subject","{$sent}");

if (!$sent) {
echo '<input name="newsletter-email" type="text" id="wo" />
<input type="image" name="newsletter" id="wogo" src="img/wogo.png" />';
}
elseif ($sent) {
echo 'Sent';
}
?>

</form>

Change the e-mail address in the code and that's it.

I receive a blank email but the 'Sent' text does not appear.

Try this,

<form action="" method="post">
<?php
$sent = $_POST['newsletter'];
if($_POST) {
mail("you@yoursite.com","Subject","{$sent}");
}

if (!$sent) {
echo '<input name="newsletter-email" type="text" id="wo" />
<input type="image" name="newsletter" id="wogo" src="img/wogo.png" />';
}
elseif ($sent) {
echo 'Sent';
}
?>

</form>