Very Simple solution. Take a water sample to any aquarium store or even a pet smart. Let them do a free water test for you and tell ya what to do
It's the best way to learn and understand how to maintain your tank. Most of the places I've been the people are very helpful and know what they are talking about
Then when the hook you you'll be up to a 100+ gallon salt tank in no time!
AND what Ray said ^ but still go talk to somone