How do you get a girl to show your breasts?

you make her take her shirt off

Don't "make" her do anything. That's terrible. You could ask her, or run your hand up under the edge of her shirt while you're making out and see how she reacts. If you're not at the point where you're making out, she probably won't show you her boobs, though.

if shes wearing a button shirt, when shes not looking just unbutton them its not rude or anything and then go around them and unbuton her bra