$.ajax({
       url: 'yourusernamecheckpage.php',
       type:'post',
       data:{'username':$('#yourusername').val()},
       beforeSend:function(){
            //do a loading msg..like checking..

       },
       success:function(msg){
       if(msg == 'ok'){
        //username available
       }else{
        //username not available
       }
    },error:function(msg){

   alert(msg);
   }


    });

//yourusernamecheckpage.php
//you can get the username as $_POST['username']:
//do your validation and
echo 'ok';exit;
//if available