Держи
<html>
<script type="text/javascript">
function Func(id_sel, id_link) {
var n = document.getElementById(id_sel).selectedIndex;
document.getElementById(id_link).href +='?mm=';
document.getElementById(id_link).href += document.getElementById(id_sel).options[n].value ;
}
</script>
<body>
<select id="sel" name="mm" class="inputs" style="width: 100px;" onchange="Func(this.id,'link')">
<option value="1" selected="selected">January</option>
<option value="2">February</option>
<option value="3">March</option>
<option value="4">April</option>
<option value="5">May</option>
<option value="6">June</option>
<option value="7">July</option>
<option value="8">August</option>
<option value="9">September</option>
<option value="10">October</option>
<option value="11">November</option>
<option value="12">December</option>
</select>
<a id="link" href=somelink.php>Some Link</a>
</body>
</html>