have an event handler read a value in a html list and display some html on this condition

Apologies my experience with html and javascript is limited so I am trying to figure out the best way to do this. Essentially I am dealing with some html that creates a form for users to send messages. The form allows the selection of the subject and text to be input.

                    <td>Subject</td>
                    <td>
                        <select id="subject">
                            <option selected="selected" value="none">Please select a subject</option>
                            {% for subject in MESSAGE_SUBJECTS %}
                                <option value="{{subject}}">{{subject}}</option>
                            {% endfor %}
                        </select>
                        <!-- <input type="text" id="subject" size="80" value='No Subject' onfocus="if(this.value==this.defaultValue)this.value='';" name="subject" /> -->
                    </td>
                </tr>
                <tr>
                    <td>Message</td>
                    <td><textarea id="newmessage" cols="80" rows="6"
                            onfocus="if(this.value==this.defaultValue)this.value='';"
                            name="newmessage">Please type your message here</textarea></td>
                </tr>

Without going into too much detail the subjects are just a series of strings. For the purposes of this we can just call them ‘sub1’ and ‘sub2’. I want to make it so the textarea only appears when ‘sub1’ is selected from the dropdown list.

I believe a javascript event handler is the way to go here and there is an associated javascript file with some functions but I can’t figure out what the syntax would look like. I assume it is something like:

function conditionalMessage() {
    if( subject == "sub2" ) {
        //some code that defines displaying text to explain now message can be sent with this subject
}   else {
        //some code that displays the message box as it currently does
}
 

Is this a case of defining the message box and the text response as divs and then calling the divs?

This is a Django based system.

Thanks for any and all help