like this:
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>Untitled Document</title>
<script type="text/javascript">
function setTextField(theVal) {
var theText = document.getElementById("textfield");
if(theVal == "y") {
theText.style.display = "block";
} else {
theText.style.display = "none";
}
}
</script>
</head>
<body>
<form name="form1" method="post" action="">
<p>
<label>
<input name="RadioGroup1" type="radio" id="RadioGroup1_0" onChange="setTextField(this.value);" value="y" checked>
Yes</label>
<br>
<label>
<input type="radio" name="RadioGroup1" value="n" id="RadioGroup1_1" onChange="setTextField(this.value);">
No</label>
<br>
</p>
<p>
<input type="text" name="textfield" id="textfield" style="display: none;">
</p>
</form>
<script type="text/javascript">
setTextField("y");
</script>
</body>
</html>