Getting values from database in drop down using AJAX

[#1]

I am having 2 dropdowns what I want is to get values from database when I select any value from 1st drop down the 2nd drop Down should change values and gets populated from database according to the value selected in 1st dropdown..It a one table one only and I am using JavaScript and I know that I need to use ajax but have no idea how should I connect to database through that..

<label> List of Tables : </label><br>

<form name="myform" id="myForm">

   <select name="optone" id="jobSelect" size="1">

   <option value="" selected="selected">Select job</option>

   </select>

   <br>

   <br>

   <select name="opttwo" id="attrSelect" size="1">

   <option value="" selected="selected">Select attribute</option>

   </select>

</form>

<script> // AJAX Implementation
function showJobs() {

   str = document.getElementById("jobs").value;

   str1 = document.getElementById("attributes").value;

   if (str == "" || str1 == "") {

   document.getElementById("txtHint").innerHTML = "";

   return;

  }

   if (window.XMLHttpRequest) { // code for IE7+, Firefox, Chrome, Opera, Safari
   xmlhttp = new XMLHttpRequest();

  } else { // code for IE6, IE5
   xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");

  }

   xmlhttp.onreadystatechange = function () {

   if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {

   document.getElementById("txtHint").innerHTML = xmlhttp.responseText;

  }

  }

   xmlhttp.open("GET", "WHAT TO MENTION HERE + str + "&q=" + str1, true);

   xmlhttp.send();

}

</script>

PLEASE SUGGEST SOMETHING!!!!!