Shopping cart blues

[jumpink]

I have a client side shopping cart. The problem is with my form. If someone does not choose a size from the drop down menu it will still add to cart. I want an error message to pop up if no size is chosen. Here is my form

<form action   = "javascript: void 0;"
      method   = "post"
	  onsubmit = "SetDesc ('shirt');
                  SetCode ('ts00014');
                  SetImg ('images/cartimage/shirt.gif');
                  SetPcat (2);
                  if (ReadForm (this, true))
                    CallView ();
                  ClearAll ();
                  return false;">

  $17.95 - 
  <input type="hidden" name="initer" value=" @0" />
  <select size="1" name="size">
 
    <option selected value="Select a size">Select a size</option>
    <option value = "Small @17.95">Small</option>
    <option value = "Medium @17.95">Medium</option>
    <option value = "Large @17.95">Large</option>
	<option value = "X-Large @17.95">XL</option>
	<option value = "2XL @19.95">2X ($2)</option>
  </select>
  <input type    = "submit"
         name    = "submit"
         value   = "add to cart" />

<br /><br />
Input <b>Quantity</b> to order - 
  <input type  = "text"
         name  = "qty"
         value = "1"
         size  = "4" />
<br /><br />
</p>
</form>

I found a form validation script at http://www.dynamicdrive.com/dynamicindex16/requiredcheck.htm"]

http://www.dynamicdrive.com/dynamicindex16/requiredcheck.htm

[/URL]
Not sure how to customize it to work in my script. Here is my script http://www.precisemessenger.com/private/cart1.js"]

http://www.precisemessenger.com/private/cart1.js

[/URL]
Any help is appreciated.
Thanks