First of all, congratulations. Second, don't worry about your ring yet; you can go together to get that sorted out after you've popped the question. I'll assume that you have talked with your special someone about what she would like in a ring before, so I won't go into types or styles of rings.
However, I cannot stress enough what you said about staying away from mall stores. A good jewelery store will NEVER pressure you. They will let you leave and think about a purchase (they might let you know they won't hold a ring for you, but no pressure). In fact, I highly recommend leaving every store to "sleep" on your ring choice before buying, that way you can reflect on how much it will cost and you can see if they pass the "no pressure" test.
Listen to every store's sales pitch. Let them know that you are shopping for an engagement ring and that you don't know much about it. Better stores will go into detail about diamond certification and the three "C"s. If you listen to several stores pitches, you'll get a good idea about how to pick a ring/stone and you'll also get a good idea of which stores were not telling you everything.
A good jeweler will let you pick out the ring and the central diamond separately. The jeweler should offer to show you several diamonds of different quality and sizes in your price range and will offer to let you see them through a microscope. Stay far away from rings that already have the diamond in the ring. Also, stay far away from any store that rates their diamonds "in house". The diamonds should be certified by a well known and outside source.
Finally, if you can find it you want some sort of lifetime warranty and maintenance included with the ring.
All that being said, I'd look in moderately priced local shopping areas that are old-fashioned-down-town-styled for local jewelers that have been around for 30 years or so. If you can find a family owned store that's been around for generations then that's even better. It may be cliche, but look in your yellow pages for ads (probably the smaller ads). Ask your friends and family for recommendations too.
Good luck.