Ajax sign up and login

if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
		alert("blahblah");
    }
  }
xmlhttp.open("POST","site.com/signup.php",false);

xmlhttp.send("username=" + document.getElementById("username").value + "&password=" + document.getElementById("password").value);

}

Trying to send sign up info to mysql table. What else do I need apart from above and how do I then use this in a login. (without using jquery).