Where to add Javscript Function and how

<html>

<head>

<script type="text/javascript">

function myfunction()

{

alert("HELLO")

}

</script>

</head>

<body> <form> <input type="button" onclick="myfunction()" value="Call function">

</form>

<p>By pressing the button, a function will be called. The function will alert a message.</p>

</body> </html>