There are many modules available for creating a popup block in Drupal7. But it is much simpler to programmatically create a popup in Drupal using the jquery modal.

First, we have to load the external js file for the jquery default modal structure. 
Paste this code to your template.php

function MYTHEME_preprocess_page(&$variables) {

  drupal_add_js('https://code.jquery.com/jquery-1.12.4.js', 'external');

  drupal_add_js('https://code.jquery.com/ui/1.12.1/jquery-ui.js', 'external');

}

Then add a block inside the site with class name as popup and add your content

Then add a custom js file and paste this code

(function ($) {

  $(function () {

    $(".popup").dialog({

      modal: true,

      autoOpen: false,

      // custom close button

      buttons: {

        "X": function () {

          $(this).dialog("close")

        }

      },

      // for aligning block in center

      my: "center",

      at: "center",

      of: window,

      // effects to show the popup

      show: {

        effect: 'fadeIn',

        duration: 500

      },

      hide: {

        effect: 'fadeOut',

        duration: 500

      }      

    });

    // popup on a specific page load

    if (window.location.pathname != ('/test')) {

      $(window).load(function() {

        $('.popup').dialog('open');

      });

    }

    // popup on a button click

    $("button").click(function () {

      $('.popup').dialog('open');

    });

  });

})(jQuery);

Style your popup block as your desired design.