First of all, here is the javascript function that tests if an email address is valid.
It checks for a lot of things: to see if the email contains a "@" sign and a dot, checks to see if the @ is before the dot, and so on.

<script>
function isMail(str)
{
   var at="@";
   var dot=".";
   var lat=str.indexOf(at);
   var lstr=str.length;
   var ldot=str.indexOf(dot);
   if (str.indexOf(at)==-1) return false;
   if (str.indexOf(at)==-1 || str.indexOf(at)==0 || str.indexOf(at)==lstr) return false;
   if (str.indexOf(dot)==-1 || str.indexOf(dot)==0 || str.indexOf(dot)==lstr) return false;
   if (str.indexOf(at,(lat+1))!=-1) return false; if (str.substring(lat-1,lat)==dot || str.substring(lat+1,lat+2)==dot) return false;
   if (str.indexOf(dot,(lat+2))==-1) return false;
   if (str.indexOf(" ")!=-1) return false;
   return true;
}

// the next function will be used to validate the form. I made this as a separate function because you might want to validate more fields.
function validate_form()
{
   if (!isMail(document.form1.mail.value))
   {
      alert("Please enter a valid email address");
      document.form1.mail.focus();
      return false;
   }
   // if the value entered is not a valid email address, 
   // we show a message, focus on the field and return false.
   // returning false will make sure the form is not submitted

   // if we got here, it means the email address is valid
   return true;
   // return true means that the form is submitted.
}
</script>

After placing this function in the code, here is what you need to put in the form.

Let's say the form only has the email address and a submit button.

<form action="/" method="post" onsubmit="return validate_form()" name="form1">
<input type="text" name="email" value="">
<input type="submit" name="submit_button" value="continue">
</form>