jquery php ajax two select box

I am quite new to this jquery ajax. What I want to know is how can I populate the content of my second select box based on the selection from the first select box. Say here is my HTML:

Code:

---

<form>
<table>
<tbody>
<tr>
  <td>
    <select id="first" name="year">
      <option value="1">1</option>
      <option value="2">2</option>
    </select>
  </td>
</tr>
<tr>
  <td>
    <select id="second" name="section">
      <option value=""></option>
      <option value=""></option>
    </select>
  </td>
</tr>
</tbody>
</table>
</form>

---

The content of my second select box will be populated from a mysql results of WHERE value of first select box. I can build the PHP file, I'm just confuse on how to return and populate it on the select.

Any leads to tutorials or samples? Thank you so much.