I want to make a drop down box do this I do not want to use check box anyone just when they make a selection

THXS Steve

<input name="cruise_air" value="true" onclick="cruise_air_check()" type="checkbox">


How would i modify so when they chose one from the list it will do the above

<select name="cruise_departure_city" value="" size="1">
<option selected>Select Air Port</option>
<option value="atl">Atlanta</option>
</select>