I would recommend you check out our SW
articles section. You are going to need something to host beneficial bacteria (99% of people use Live rock for this) and you will need to cycle the tank (also an article).
That canister filter will need to be cleaned often so that stuff doesn't build up in there.
You will want to eventually ditch the hydrometer for a refractometer, the hydros are really inaccurate.
You should go get ammonia, nitrite and nitrate test kits and check what your levels are at now and through the cycle. There is so much we could tell you right now (not that you have done anything wrong) so I would suggest you do some reading into the basics and then get back to us with any further questions.