That will be the reason why all your fish died - ammonia poisoning. Yes, your LFS will test for you, make sure they tell you the actual number, not "yeah, that's fine" - anything other than 0ppm is NOT fine. You can also buy your own chemical test kits. I use Salifert, the individual tests for ammonia, nitrite and nitrate will set you back about £35 from Amazon. You can also buy the API master test kit for around the same, which gives you various other tests that you won't really need.
You need to properly cycle your tank. This means growing the beneficial bacteria in your tank and filter to process the ammonia the fish produce into nitrite and then to nitrate. To do this, the bacteria need a source of ammonia, either bottled ammonia or fish. All the experts on here recommend doing it with bottled ammonia, and since you have an empty tank right now, this is an option for you.
If you consult the Cycle My Tank subforum here, there are threads pinned at the top which go into detail.
There is a really good shortcut - if you know someone else with a mature filter, swap 10-20% of their mature media with your immature media. The bacteria will recolonise your immature media, so they won't notice a difference, and you will have a head start with some bacteria that will quickly grow in your filter.