You need to put your aquarium through what is known as a cycle. Simply this refers to the cycle where organic waste turns into ammonia, the bacteria turns this ammonia into nitrite and the bacteria then turns this into nitrate which is absorbed by the plants or removed via water changes. This can be done with fish but may lead to toxic levels of ammonia, nitrite or nitrate in the aquarium so commonly it is done by introducing fish food or ammonia to the aquarium. This is known as a "fishless" cycle and is much safer.