Help.. My Dalmation Molly's are dying !?

Help.. I'm new to owning fish (other than golfish).. I just bought a 5 gal tank.. I tested the water and waited to to purchase fish until the water levels were ok.. I later bought 4 mollys and 2 upside down catfish.. Well it's been 2 weeks and I've lost 2 of my male mollys and am scared to death that I'll lose the rest.. I keep going to the pet store where I bought the fish and have my water tested and found out the levels are now through the roof.. Yes, I have now learned that I had to many fish for my 5 gal tank.. problem is.. No matter what I do.. the levels in my tank won't go down. I bought testers but don't understand the NO3/PH balances and what they mean ? I'm about to cry.. I never intended to kill these fish and I just want to do EVERYTHING I can to keep them alive. What is cycling ?? I asked the pet shop people and they gave me a blank look (got a bit nervous).. Can my catfish survivie in brackish water ? Will buying a bigger tank solve this problem ? Could putting salt in the tank help my fish ? I need any and ALL help....