how to build an ask/answer website with html/css/ajax/php [on hold]

//this is register.js       

window.onload=initPage;
    var request;

    function initPage(){
        var submit=document.getElementById("submit");//<input type="submit" id="submit>"

        submit.onclick=getRegister;
    }

    function getRegister(){
        request=createRequest();
        if(request==null){
            alert("unable to create request");
            return;
        }

        var url="register.php";
        request.open("GET",url,true);
        request.onreadystatechange=checkAccount;
        request.send(null);
    }

    function checkAccount(){
        if(request.readyState==4){
        if(request.status==200){

        }
        }
    }


    function createRequest() {
      try {
        request = new XMLHttpRequest();
      } catch (tryMS) {
        try {
          request = new ActiveXObject("Msxml2.XMLHTTP");
        } catch (otherMS) {
          try {
            request = new ActiveXObject("Microsoft.XMLHTTP");
          } catch (failed) {
            request = null;
          }
        }
      } 
      return request;
    }

I have a register.php,a register.js,a register.html and with a four input:account,password,nickname,email. I have to check whether the account is already in the database,I know how to interact with mysql using php,the confusion is I don't know how to interact js and php,and where to compare the account,could you please show me the code examples?Thank you.(I'm trying to build a website such like stackoverfolw but not that awesome...I'm trying to learn by myself..)