hi and welcome ! You also may want to get some water testers of your own, I recommend the
API brand, to test ammonia, nitrite, ph and nitrAte. If the pet shop doesn't have the API brand, just make sure you ask for
liquid test kits , not the paper strips, as the paper ones aren't accurate.
also, when you do the water changes, make sure that the new water you're adding in is dechlorinated, and is the same (or very close to) the existing water in the tank. Water temperature fluctuactions , as well as ph fluctuations, can stress or even kill fish, so try to be careful about that. You may want to pick up a thermomenter at the pet shop just for this, as it will make things a lot easier.

Also, the ph
test kit.
here's an informative link that explains the
nitrogen cycle of aquariums. It provides great info for beginner fishkeepers!
http://www.fishlore.com/NitrogenCycle.htm
sorry for the fish losses, but hopefully you'll have no more. Just keep diligent with the water changes , as Shawnie suggested !
Good luck with your new tank !