Been asked a million times before mate, there is a section dedicated to it which you would have been better off posting in.
Basics are = Ross Tech are the people who make the official stuff and the software is free then you need to buy the cable which is the expensive part £200+ I believe and the cable is the bit that acts as the licence key by unlocking the software.
Cheap eBay cables are notoriously flaky and don't offer any support or the full access to the software, in short they are cheap for a reason.
If all you want to do is read and clear a few codes go for the cheapo eBay cable, if you want to get access to everything go legit.
And for everything else there's the Search feature