As stated, the tank needs to be cycled. The important thing is to test the water, with a reliable kit. Most people use the API Freshwater Master Test Kit. It is cheap on ebay or amazon, but someone expensive in stores (by comparison).
If you don't know how to cycle a fish tank, read these, they will help, along with Masha's links:
http://www.aquariumadvice.com/forums/f12/fish-in-cycling-step-over-into-the-dark-side-176446.html
Tips and Tricks For Your Fastest Fishless Cycle! - Aquarium Advice
The first one is about fish in cycling, the second is about fishless cycling. To cycle a tank you need to get some level of ammonia, and have bacteria in the tank (that get there through the air) to "eat" the ammonia, and make nitrites, which is "eaten" by another type of bacteria that produces nitrates. Ammonia and nitrites are very bad for fish, and levels over .25 ppm of those are bad and can cause permanent damage (given enough time in that water). Nitrates are much less toxic. Nitrates should be under 40 ppm, but under 20 ppm is best. Nitrates are removed through water changes.
A fish's waster produces ammonia, which is the source for fish in cycling. For fish in, you just add fish, test daily, and do a 25 % water change when water gets above .25 ppm for ammonia or nitrites, until you consistently get 0 ammonia, 0 nitrites, and x nitrates. Then, you are cycled (you have built up enough bacteria to "consume" the amount of waste produced by your current stock of fish. This is known as a bioload, the amount of waste a fish produces), and can add more fish, a few at a time.
Fishless cycling uses a source of ammonia (pure ammonia [bottled], a raw shrimp [the kind we eat], or fish food), try to get ammonia up to 4 ppm (but less is okay), and then just add more ammonia when it drops under 4, and do a water change when ammonia or nitrites somehow get over 5 ppm. Once you consistently get 0 ammonia, 0 nitrites, and x nitrates, you are cycled, and can add your whole stock at once. If you don't add your whole stock at once, say add only 5 fish, and then wait a week, you cannot add your whole stock then, as the bacteria will die off until there is just enough ammonia produced for all of the bacteria to eat. While fishless cycling you should test at least
every other day.
Hope this helps, and welcome to AA!