You don't need to buy anything to reduce nitrites, just perform some water changes and don't forget to add your dechlorinator to the new water.
Either your tank wasn't fully cycled before you added fish or you were too vigorous cleaning or replacing your filter media.
Prime will temporarily bind harmful ammonia and nitrite for 24 hours but it is not a long term solution.
Since you are new to testing with the API test kit, be sure to follow the instructions exactly for the amount of water in the test tube, the proper number of reagent drops and the amount of time to wait to read the color.