The problem is that the tank hasn't fully cycled. The fish produce waste (ammonia) and your tank lacks enough bacteria to convert that ammonia into nitrites. There is another bacteria that will convert the nitrites to nitrates. The nitrates are then removed by water changes and to a much, much, lesser degree by plants.
I'm not sure what fish store you are going to, but they should have told you about cycling your tank first. There is some good information on this site about cycling your tank. I stole these from the signature of another member:
http://www.aquariumadvice.com/articl...cle/Page1.html
I just learned about cycling but I already have fish. What now?!
Water changes on a 29 gallon should be fairly easy to do. Take off the top and use a small contain to empty the water into a larger bucket. Empty the bucket. Refill the bucket from the tap (making sure the water is as close as possible to the tank water in temperature)...add Prime (or some other dechlorinator). Use the small container to empty the newly refilled bucket into the tank. Voila!
You'll need to do this to keep the ammonia and nitrite down until the cycle finishes. You'll know when the cycle is finished as you will start getting consistent readings of zero ammonia, zero nitrites, and your nitrates will keep going up.
Once the cycle is complete you will need to do either weekly, bi-weekly, or monthly water changes. Those will depend on what you have in your tank, and how quickly your nitrates increase.