How many fish do you have and how large is the tank? Did any fish already die and if so what do they look like? If they die with their mouth agape, it was ammonia. If not
they are dying from parasites. The parasites like ich or velvet cover the gills so the fish can't get enough oxygen out of the water. It is very common in a brand new tank, almost guaranteed.
If it's ammonia, that is easy, you put in to many fish to early.
Many people quarantine fish before they put them in a new tank and many people do something else. You have a lot of reading to do because there are a few methods to follow.
The only thing you can do at this stage is put the fish in a fresh water dip. There is a lot of information about that on here if you search for it. But basically you put the fish in fresh water for about 3 or 4 minutes if they can take it. Just be sure the water is the same temperature.
Of course if you do that, you will also need a spare tank to put them in to treat with something like copper. There is also a lot of information on that here.
Now that all of your fish are at the stage of breathing heavy they may be able to treat them in your tank.
You should start reading how to set up a new tank and all you can on parasites, immunity, feeding etc as that will be a lot to learn but don't dispair . As I said, it is common in a new tank if you don't follow certain rules.