[PHP] Need help on the list menu

I need a list menu , one is the weight of the product, when user selects the weight the Price of the product should be displayed. I’m generating the list dynamically from the DB.

How to access that particular selected value in the option.
On what event it should be written. Where should it be called.

Please help
Here is my piece of code

The dropdown displays with all the values.

<select name="wtlist">
       $result1=mysql_query("select * from prodsize");
       while($row = mysql_fetch_array($result1))
                            <option value="weight"><?php echo $row["prodwt"]; ?></option>