Onchange submit form - need help

<td width="1000" align="right">
<div class="form_content">
<form name="newad" method="post" action="/newad.php?step=1">
	<label>{""}</label><br><br>
	<select id="category" name="category">
	<option value="">{"Novo anúncio, seleccione uma categoria.."}</option>
	{foreach from=$categories item=v name=cat}
		{if $v.parent==1}
		<optgroup label="{$v.str}{$v.name}">
		{else}
		<option value="{$v.id}" {if $tmp.category==$v.id}selected="selected"{/if}>{$v.str}{$v.name|escape:"html"}</option>
		{if ( $smarty.foreach.cat.index!=$categories|@count-1 && $v.parent_id!=$categories[$smarty.foreach.cat.iteration].parent_id) || ($smarty.foreach.cat.index==($categories|@count-1) && $v.parent_id!=0)}
		</optgroup>
		{/if}

		{/if}
	{/foreach}
	</select>


<div class="buttons">
    	<button type="submit" name="Choose_categ" class="positive">
        	{"Criar anúncio"}
    	</button>
	</div>
</br>
</br>
</div>
</td>
</pre>

Can anyone help me please, thanx in advance.</br></br></div>