You can buy the test kit at Petsmart, but it will cost you about twice as much as it would if you bought it from an online aquarium supply store like that Drs. Foster and Smith site.
It's important to realize that water quality is not something you can measure just by looking at it. Your water can be completely deadly to your fish and still look just fine. The test kit is really the only way of knowing how much toxic stuff you've got in your tank. Since you set up the tank only recently, there are no bacteria present to convert ammonia to nitrite and then to nitrate. Even tiny amounts of ammonia from rotting fish food or your fishes' waste can build up to deadly amounts in a matter of days.
I would do a 50% water change on your tank ASAP. Use water that's the same temperature as the water in the tank and make sure to use a good water conditioner that removes chlorine, chloramine and heavy metals. You likely have a lot of ammonia and possibly some nitrite in your tank right now so a water change is really helpful. I would then do another 50% water change in two days. After that, 20% changes every two days will be helpful until the cycle is complete. At that point, you really only need to do 20% changes or so once a week. Keep in mind that all of these are really ballpark estimates - without that test kit, it's really impossible to tell where your water parameters are and thus when to do water changes and how much. You can also take a sample of your aquarium water to a fish store and have them test it for you. Most will do this for free. Make sure they use a good liquid test kit or electronic probes and not the paper strips, and have them give you specific numbers for pH, ammonia, nitrite and nitrate.
I know a lot of this is overwhelming, but once you get te hang of it you'll be keeping happier, healthier fish that will be more enjoyable. Good luck!