try this code
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<Script>
$(document).ready(function(){
$("#postcontent").click(function(e) {
$.ajax({type:"POST",url:"add_new_post.php",data:$("#postcontent").serialize(),beforeSend:function(){
$(".post_submitting").show().html("<center><img src='images/loading.gif'/></center>");
},success:function(response){
//alert(response);
$("#return_update_msg").html(response);
$(".post_submitting").fadeOut(1000);
}
});
});
});
</script>
<form name="postcontent" id="postcontent">
<input name="postsubmit" type="button" id="postsubmit" value="POST"/>
<textarea id="postdata" name="postdata" placeholder="What's Up ?"></textarea>
</form>