Start using JQuery
(JQuery).
Here is the javascript:
$(document).ready(function(){
// On Select option changed
$("#someId").change(function(){
// Check if current value is "audi"
if($(this).val() === "audi"){
// Show input field
$("#textInputId").show(); //This changes display to block
}else{
// Hide input field
$("#textInputId").hide();
}
});
});
Here is the HTML
<select id="someId">
<option value="volvo">Volvo</option>
<option value="saab">Saab</option>
<option value="mercedes">Mercedes</option>
<option value="audi">Audi</option>
</select>
<input type="text" style="display:none;" id="textInputId" name="someName"/>
Here is the example on JSFiddle: