There are 2 big & lethal problems you have:
- You tank hasn't been cycled
As mentioned before, your tank needs to 'cycle' before you can add any fish to it. During this period, the chemical composition of the water goes through a few stages. This takes usually at least 3-4 weeks. Only after that, you can slowly start to add fish (also don't add all the fish at once!). More information can indeed be found in this answer: How does one effectively "cycle" an aquarium?
- Your tank is overstocked
You have a small tank, and a lot of fish in it. Basically a tank that size is (in my opinion) just large enough for a single betta fish, definitely not for 10 fish.
Your fish produce waste, and if there is too much waste is also not healthy for your fish.
So the combination of these 2 things probably killed your fish.
As an emergency solution you should now:
- Do a partial water change every day (around 50% of the water)
- Add chemicals to decrease the ammonia levels in the water (ask your local pet-shop)
- There are also chemicals to speed up this 'cycling' process. It's usually called something like 'Filter Start'. Get some of those as well.
Another word of advice: don't go to your local pet shop for advice on fish for your tank. Pet shops usually don't know much about aquariums either, and only want to sell fish/equipment. Go to a specialized fish shop and ask them for advice. These specialized shops actually know what they are talking about, and normally also care about the well being of the fish they sell.
And also do your own research beforehand.