I have a problem with a javascript error: $("#slider") is undefined

How can i solve this problem?

<script type="text/javascript">
$(document).ready(function() {
    $("#slider").easySlider({
        controlsBefore: '<p id="controls">',
        controlsAfter: '</p>',
        prevId: 'prevBtn',
        nextId: 'nextBtn'
    });
});
</script>

This is my html

<div id='slider'>
    <table>
        <tr>
            <td width='325'>hello</td>
            <td width='325'>hello</td>
    </table>
</div>