Adding New Row With Forms

I need someone to point me to the write direction here. I know nothing of javascript, and I've tried looking everywhere I can on Google.

I have a drop down of all the sizes I have in a database. I call them to a form with the ID number as the value. What I want is when a user selects a size, it gets added to the table. For example:

<table>
<tr>
<td>Paper</td><td>size</td><td>quantity</td>
</tr>

<!--Repeat this bit with javascript according to the Paper/size chosen on the drop down -->
<tr>
<td>Glossy</td><td>4x6</td><td><input quantity form></td>
</tr>
<!--End repeat here -->

</table>