Hi and welcome Brittany. We need to know how exactly you set up the tank, what sort of filter it has, how long it has been running, and what sort and how many fish do you have?
I suspect you went and bought the tank, filled it with water, plugged in the equipment and then added a whole pile of fish. Am I right? If so, they are dying from ammonia poisoning.
Before going into more detail we need to know if we are on the right track (saves a lot of typing if we aren't )