To place square like that, I would suggest looking at how other squares are placed. For example, in the folder 'graphics' under 'pictures' you find 'bagSel.png'. This is the kind of square you're looking for, right? See how it is used, and try to copy that.
Ps. I have no experience with this thing at all (so I'm like a Ralts without Confusion, just growling about...)
EDIT: Oh wait, I think I misunderstood your question. You want information in those circle-thingies. Uhm. I can see that there is information displayed. Maybe try to find out how that is displayed, then copy the code for that, and display the name and quantity instead. The name is (I think) already a string (text-variable), so you can just display it. As for the quantity, that will probably be an integer (number-variable). You may have to cast it (but I don't know how that works in Ruby).
dexter's advice is basically correct. You want to add some text to your screen. Is there an existing example of text being on the screen? The item description, you say? How does that work? Can you copy it?
That's the part that updates the description text when you change which item you're looking at. However, that's just half of it. The other half is creating the window to put the text in (the window is called @sprites["itemtextwindow"]). You'll need to create two more windows, one for the item's name and one for the quantity.
Here's how the item description's window is created: