I put together an overly simplistic jsFiddle demo of how a popup is made. Here is the magic:

//Display Popup method - you can modify this to add parameters 
function displayPopup(event) {
        //dynamically create a div, button, and inner text
        var p = document.createElement('div'),
            b = document.createElement('button'),
            t = document.createTextNode("Text and stuff");

    //add popup class to newly created div
    p.className += 'popup';
    //Add text to close button element
    b.innerHTML = "close";
    //Append text to popup div
    p.appendChild(t);
    //Bind click event to button element
    b.onclick = closePopup;
    //Append button to popup div, and append popup to document body
    p.appendChild(b);
    document.body.appendChild(p);

    p.style.display="block";

    //This function hides the popup
    function closePopup(event){
        p.style.display="none";
    }
}

It should by no means be considered complete or production material, but I hope it helps you get started on the right path to extending it to achieve the functionality you're looking for.