Inside the blue box is where you do editing.  The area you're editing is shown on the left, in this case, the front of a t-shirt.

Note that the shirt is for a girl right now.  Click on the "shirt options" tab at the upper left of the blue box, you can change to a men's, women's or baby's shirt.  You can also choose from a variety of styles and colors.

Now, take a closer look at the edit pane.

The top box first. At the top of the box, the "Add images" and "Add text" buttons allow you to add your own images and text.

Below that, you edit text. The text already on the shirt is "your text here", shown under the "Add image" button. To change or delete it, click "change text" or "delete" under the "Add text" button.

To move the text, use arrows on the left.
To change size, color, alignment or font, use buttons in the middle.
To center vertically or horizontally, use buttons on the right.

The bottom box shows a thumbnail of the graphic. Click on it to edit the graphic. The box will expand.

You can change or delete the graphic by clicking "change image" or "delete" at the upper right corner of the blue box.

To move the image, use arrows in the middle of the box.
To make the image bigger or smaller, click the magnifier buttons.
To rotate the image, click the bent arrows.

You can also center the graphic vertically or horizontally using buttons at the lower right.

Once you're happy with the look of the design, you're done with the front of the shirt.

Now you can switch to the back of the shirt by clicking on the pictured inside the blue box.  Right now the back has nothing on it.  But you can add images or text as described above, or you can leave it blank.

When you're all done, click "add to cart" to put your custom shirt in the shopping cart.