I've introduced you to a lot of JavaScript.
Now would be a good time to review what I've covered.

Functions

alert, setTimeout, getAttribute

Other Important Stuff

this, variables, if statement, own functions, %

Examples
  var card1='';
  function card_clicked(image) {
    var n=image.getAttribute('n');
    image.src='cards/'+n+'.png';
    if (card1=='') {
      card1=image;
    } else {
      var n1=card1.getAttribute('n');
      n1=n1%13;
      if (n1==n2) {
        setTimeout(clear,1000,card1);
      }
      card1='';
    }
  }
  function clear(image) {
    image.src='cards/0.png';
  }

There's no point in moving on until you're totally comfortable with all the ideas on this page.