I assume you have a custom block with a form created from your custom module.
You would need to add an attribute #prefix to the first field item starting from which you want to hide the items and add an attribute #suffix to the last field item you want to hide, For example,
$form['first_item']['#prefix'] = '<div id="accordian">';
// more elements here
$form['last_item']['#suffix'] = '</div>';
Then, you can bind jQuery.slideUp() and jQuery.slideDown() to the more/less link element, let's say for example, #moreless.
$('#moreless').bind('click', function(){
if( $('#accordian:visible').size() ){
$('#accordian').slideUp();
$(this).html('More »');
}else{
$('#accordian').slideDown();
$(this).html('Less «');
}
});
You can see the working jQuery code here. These two jQuery functions are provided by jQuery core API. You don't need to include any extra library.