This is simple! I have a such script on my site. Here's the source code, simply change options1-4 and text1-4.
HTML Code:
<table>
<tr>
<td><form name="ddmessage">
<tr>
<td><select name="selectbox" size="1" onChange="changecontent(this)">
<option selected value="options1">options1</option>
<option value="options2">options2</option>
<option value="options3">options3</option>
<option value="options4">options4</option>
</select>
</td>
</tr>
<tr>
<td><textarea rows="8" name="contentbox" cols="35" wrap="virtual"></textarea>
</td>
</tr>
</form>
</td>
</tr>
</table>
<script language="JavaScript">
var thecontents=new Array()
thecontents[0]='text1'
thecontents[1]='text2'
thecontents[2]='text3'
thecontents[3]='text4'
function changecontent(which){
document.ddmessage.contentbox.value=thecontents[which.selectedIndex]
}
document.ddmessage.contentbox.value=thecontents[document.ddmessage.selectbox.selectedIndex]
</script>