You need a "differential" amplifier. It can be a single precision opamp, or made from multiple opamps, or a ready made "instrumentation amplifer". These can be bought from Analog Devices, Linear Technology, Texas Instruments, National, Fairchild, and about fifty other makers. For starters, look up the data sheets and application notes for the INA12x (1<x<9) family of Instrumentation Amps.