-12v Regulator selection & conversion
If you do not have a negative voltage input, then you will need some sort of DC to DC converter to generate it.  Depending on how much current you need, you might be able to get away with a simple charge pump, or you may need something a bit more complicated.  You can generally buy complete DC to DC converter modules that you can solder on to your board if you don't care to design one yourself.  

As for rescaling the signal, there are a few ways to do that.  One method that I have seen is to use a voltage divider where you hang the 'low' end to +2.5v (or similar) instead of ground.  Another method is to build a summing amplifier where you can add the input voltage (possibly scaled down in a voltage divider) with an offset to rescale and shift it into the ADC input range.  So long as the circuit maps zero, full scale low, and full scale high to points within the ADC range, you can correct any offsets in software.  Just make sure that most of the ADC input range is covered, otherwise you won't get good resolution.