OMG that's the saddest thing I've seen on here in ages...
Not putting the blame on you - sounds like you have no knowledge of fish-keeping and I can imagine you believed that the guy in the shop must surely know about fish if he's selling them to you. What a heartless, money-grabber he must be...I mean, I know times are hard on the economic front, but dicing with the lives of living creatures is so cruel.
A hard lesson learnt...it's heartbreaking knowing that 40 fish have died like this - and fish aren't cheap to buy, so I guess you're feeling rather annoyed about that, too.
Not a good way to start off in this hobby - but you've found a great forum here where people will give you guidance and help along the way. So, put all of this down to a bad experience and if you still want to keep fish, do lots of research on here first.
Get your tank properly set up and cycled, as others have already mentioned and given links, and then once you're at the cycled stage (which will be weeks away if you do fishless cycling, so patience is a must) check back on here and we can all help you to choose the correct fish and quantities to suit your tank.
Regards, Athena