Live Demo

Sounds like you might want a light box,and since you didnt tag your question with jQuery included is a pure JS example of how to make one.

JS

var opener = document.getElementById("opener");

opener.onclick = function(){

    var lightbox = document.getElementById("lightbox"),
        dimmer = document.createElement("div");

    dimmer.style.width =  window.innerWidth + 'px';
    dimmer.style.height = window.innerHeight + 'px';
    dimmer.className = 'dimmer';

    dimmer.onclick = function(){
        document.body.removeChild(this);   
        lightbox.style.visibility = 'hidden';
    }

    document.body.appendChild(dimmer);

    lightbox.style.visibility = 'visible';
    lightbox.style.top = window.innerHeight/2 - 50 + 'px';
    lightbox.style.left = window.innerWidth/2 - 100 + 'px';
    return false;
}

Markup

<div id="lightbox">Testing out the lightbox</div>
<a href="#" id="opener">Click me</a>

CSS

#lightbox{
    visibility:hidden;
    position:absolute;
    background:red;
    border:2px solid #3c3c3c;
    color:white;
    z-index:100;
    width: 200px;
    height:100px;
    padding:20px;
}

.dimmer{
    background: #000;
    position: absolute;
    opacity: .5;
    top: 0;
    z-index:99;
}

Hi now used to simple jquery code as like this

$(document).ready(function(){
    $('.login__form').hide();
    $('.click').on('click', function(){
      $('.login__form').show();
    });
  $('.closePopUP').on('click', function(){
     $('.login__form').hide();
  })
  
});

body {
    background: #a7a7a7;
}
.closePopUP{position: absolute;
right: 8px;
top: 7px;
cursor: pointer;
border: solid 2px red;
border-radius: 50%;
font-size: 14px;
font-family: arial;
padding: 2px 6px;}
.login__form {
	width: 320px;
    padding: 20px;
    background: #fff;
    border-radius: 5px;
    border-top: 5px solid #ff4e50;
    margin: 0 auto;position:relative;
}

.login__form fieldset{
	border: 0;
}

.login__form h3 {
    text-align: center;
    color: #666;
    font-size: 18px;
    text-transform: uppercase;
    margin-top: 0;
    margin-bottom: 20px;
	font-weight: bold;
    padding: 0;
    margin: 0;
    margin-bottom: 12px;
}

.login__form input {
    width: 100%;
    height: 42px;
    box-sizing: border-box;
    border-radius: 5px;
    border: 1px solid #ccc;
    margin-bottom: 20px;
    font-size: 14px;
    font-family: Montserrat;
    padding: 0 20px 0 50px;
    outline: none;
}

.login__form input#username {
    background: #fff url('http://i.imgur.com/u0XmBmv.png') 20px top no-repeat;
    background-size: 16px 80px;
}

.login__form input#username:focus {
    background: #fff url('http://i.imgur.com/u0XmBmv.png') 20px bottom no-repeat;
    background-size: 16px 80px;
}

.login__form input#password {
    background: #fff url('http://i.imgur.com/Qf83FTt.png') 20px top no-repeat;
    background-size: 16px 80px;
}

.login__form input#password:focus {
    background: #fff url('http://i.imgur.com/Qf83FTt.png') 20px bottom no-repeat;
    background-size: 16px 80px;
}

.login__form input:active, .login__form input:focus {
    border: 1px solid #ff4e50;
}

.login__form input#button {
    width: 100%;
    height: 40px;
    background: #ff4e50;
    box-sizing: border-box;
    border-radius: 5px;
    border: 1px solid #e15960;
    color: #fff;
    font-weight: bold;
    text-transform: uppercase;
    font-size: 14px;
    font-family: Montserrat;
    outline: none;
    cursor: pointer;
    margin: 0;
    padding: 0;
}

.login__form input#button:hover {
    opacity: 0.7;
    filter: alpha(opacity=70);
}
.click{float:left;cursor:pointer;}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<button class="click">login popup</button>
<form name="login" class="login__form">
<span class="closePopUP">X</span>
			<h3>Login</h3>
			<fieldset>
				<input type="text" value="" placeholder="Username" id="username">
				<input type="password" value="" placeholder="Password" id="password">
				<input type="submit" value="Submit" id="button">
			</fieldset>
</form>