the lfs will in most cases just get in your pocket
the order to do things is
1.fill tank
2.put in shrimp in mesh bag
3. when shrimp decay they will spike levels, then remove shrimp
4. get sand/rock from established tank and add to your tank
5. add live rock to your tank
6. add hermits and crabs to your tank
7. once all levels have been down for multiple days-or a week ESPECIALLY AMMONIA AND NITRITE then you are ready to go fish shopping
I am sorry to say this but your fish ARE going to die soon. I can almost assure you you do not have a sufficient level of bacteria living in your tank to dissipate the ammonia and nitrite naturally. You should buy a book too. That's probably the number one mistake beginners make. You can buy a couple bottles of Cycle, amquel, or ammo-lock right now. They will help, but are not the natural way to do this hobby. The starfish, yellow tang, and goby at a minimum should go back today if your tank is not cycled.