<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.1.1/jquery.js"></script>
<form>
<input type="text" id="username"><br>
<input type="password" id="password"><br>
<input type="button" value="submit" id="submit">
</form>

<script>
$(document).on('click','#submit',function(){
$('.error').remove();
var username=$('#username').val();
var password=$('#password').val();
var err=0;
if(username==""){
$('#username').before('<p class="error">Please enter username</p>');
err=1;
}
if(password==""){
$('#password').before('<p class="error">Please enter password</p>');
err=1;
}
if(err==0){
// send your ajax here
}
});
</script>