Definitely don't use water from your freshwater tank! Bacterial colonies that live in saltwater and freshwater are different. Like Yote said, use a piece of shrimp to cycle it. As it decomposes, the tank will cycle. Ammonia will rise, then nitrites will rise. Then nitrates will rise. Once ammonia and nitrites are back at zero, do a water change (nitrates won't drop on their own). Then you can add a clownfish. I would add just one to start. They are usually happier on their own anyways. You can try to add a second one later on if you want, but sometimes they'll fight.