ive got a cable which you can connect vag-com to it, (vag-com is the popular software used ...but pretty expensive) i just use the trial version (which is kinda half cracked, i dunno how that happened but i cant do everything but its better than just giving the usual trial readings)
read up on it they have a wiki, i bought my cable on ebay for about £8 i think.
but dont go rushing to buy something you dont know about (i do it all the time and realise what i buy is no good) read up on vag-com and ross tech in google.