On your check out page, you can change your input field to select list by changing this line:
<input type="text" name="exp_month" id="exp_month" value="" /> to
something like this:
<select name="exp_month" id="exp_month" >
<option value="2000">2000</option>
<option value="2010">2010</option>
</select>
make sure the name and the id stays the same so you don't have to make any updates on the confirm page.