It would help if you mentioned which language you are using to program it with. With LabVIEW, you have a large number of shipping examples that use DAQmx which is the driver for the USB-6008.
Like I said, there are numerous shipping examples for reading voltage. Just go to Help>Find Examples. When it's open, expnd the Hardware Input and Output>DAQmx>Analog Measurements>Voltage. A simple one to start with is Cont Acq&Graph Voltage-Int Clk. You can also use the DAQ Assistant on the NI Measurements>DAQmx - Data Acquisition palette. From the DAQ Assistant you can even scale your voltage readings into whatever you need. You can also do this in MAX or with DAQmx functions. All of this requires that you are using DAQmx and not DAQmx Base. If you don't have DAQmx, click the Support link at the top of this page and download it.