BING BING BING!!
Some girls' nipples do get harder and firmer (and sometimes even darker in color) when they're aroused. It is kinda like when guys get erections; as far as I know, it's the same basic principle (spongy erectile tissue with increased blood flow = hardness).
As for a "certain way you're supposed to feel a girl's breasts," we're all quite different. Basically, I'd advise you ask her about it while you're with her, things like, "Do you like this?" or "Does that feel good?"
Communication's going to be your best relationship tool, in bed and out of it.
~lemming, Scarleteen Advocate
want to know the inner lemming? read her diary at http://innerlemming.diaryland.com/ .