Contrary to what many people believe (and possibly hope), you can't really lose weight in a specific area, unless you go through surgery or liposuction. You can take steps to lose weight and reduce body fat all over. Chances are the belly fat will also go away with the rest of the excess (though possibly not to the extent you're hoping).

You should do some exercises on your abdominal area despite your back. Make sure you consult a professional to see what exactly works for you, but these exercises will help your posture more than anything else. It is particularly hard for women to get the "6 pack" look, and they have to have a very low body fat percentage for it to show.