When your tank is not cycled, you will get an ammonia spike followed by a spike in nitrites. That is probably the most likely. Read up on Fishless/Aquarium nitrogen cycling ... there needs to be a steady ammonia source in the tank (get pure ammonia (without scent & soaps) from a hardware store or add fish food to the tank to supply the ammonia) to help build up the population of beneficial bacteria that you need to be able to completely convert toxic ammonia and nitrite into non-toxic nitrate (which is OK as long as it's levels aren't too high.) If your fish store can give you readings for ammonia and nitrite, I would keep adding some fishfood or ammonia to the water, and have the water regularly tested until there is NO ammonia in the tank and NO nitrites in the tank and just nitrates. Then you know it is cycled and ready for fish 
Edit: I see that you have already purchased a new fish. To keep ammonia and nitrite levels down, you'll have to do large (I'd say over 50%) water changes very regularly, and test the water as often as possible (ideally you could test it daily with your own kit - the best kind are liquid, like the API Master Test Kit). Make sure to treat the water with a water conditioner to remove chlorine/chloramines and temperature match it to the tank water.
Edit: I see that you have already purchased a new fish. To keep ammonia and nitrite levels down, you'll have to do large (I'd say over 50%) water changes very regularly, and test the water as often as possible (ideally you could test it daily with your own kit - the best kind are liquid, like the API Master Test Kit). Make sure to treat the water with a water conditioner to remove chlorine/chloramines and temperature match it to the tank water.
