How can I expand and collapse a <div> using javascript?

Okay, so you've got two options here :

1. Use jQuery UI's accordion - its nice, easy and fast. See more info here
2. Or, if you still wanna do this by yourself, you could remove the fieldset (its not semantically right to use it for this anyway) and create a structure by yourself.

Here's how you do that. Create a HTML structure like this :

<div class="container">
    <div class="header"><span>Expand</span>

    </div>
    <div class="content">
        <ul>
            <li>This is just some random content.</li>
            <li>This is just some random content.</li>
            <li>This is just some random content.</li>
            <li>This is just some random content.</li>
        </ul>
    </div>
</div>

With this CSS: (This is to hide the .content stuff when the page loads.

.container .content {
    display: none;
    padding : 5px;
}

Then, using jQuery, write a click event for the header.

$(".header").click(function () {

    $header = $(this);
    //getting the next element
    $content = $header.next();
    //open up the content needed - toggle the slide- if visible, slide up, if not slidedown.
    $content.slideToggle(500, function () {
        //execute this after slideToggle is done
        //change text of header based on visibility of content div
        $header.text(function () {
            //change text based on condition
            return $content.is(":visible") ? "Collapse" : "Expand";
        });
    });

});

Here's a demo : http://jsfiddle.net/hungerpain/eK8X5/7/