Onchange Event To Disable / Enable Textbox

My question is I have this for form with couple options and a text box and what I want to do is when user selects the "none" textbox to be enabled and if user chooses "mysite" option textbox to be disabled so user can't put his/her input.

Here is my code:
Code:
<form id="form1" method="post" action="">
<label>
<select name="none" id="noneid" >
<option value="none">None</option>
<option value="mysite">Mysite</option>
</select>
</label>
<label>

Another
<input type="text" name="another" id="anotherid" />
</label>
</form>
I think I need a javascript function to trigger by onchange event to disable or enable textbox depending on the user input. But I don't know how to write that function.