You will need to cycle your tank with an ammonia source. This will take roughly a month. During this time you will measure your ammonia, nitrite, and nitrate levels. Once you have no ammonia or nitrite after adding more ammonia source you are cycled. Do a large water change to bring the nitrates down and you are good to go.
Ammonia, nitrite, and nitrate are poisonous to fish. Nitrates can go up to 40 and be "ok" if it is a FOWLR
setup. Be careful with your hydrometer, they can get air bubbles on the arm and give false readings on your salinity. I used to have 2 hydrometers that both gave different readings so I got a refractometer so I never had to deal with that issue again.