Auto Form Fill on option select

Hi all, I have a question that I can't get my head around and was hoping someone could answer for me. I have a form with a number of fields. Field one is a <td><select name="thisSteamField"></td> The option fields are built from an sql statement and a where clause builds the options fields ... <option value="<? echo $teamname; ?>"><? echo $teamname; ?></option> .... etc... When I select one of the options, the form automatically goes off and does another sql select a with the WHERE value of the selected $teamname, and then populates the next field, but I can't think how. Any pointers? TIA K