You'll need either more live rock or you can buy somebase rock to save some money. You should have at least 15lbs of rock in there. In saltwater the rock is your filter really, the other parts of the filtration system just remove particulate matter that floats around.
Your beneficial bacteria will reside in the sand and rock and convert the ammonia (fish poop and other organics) inbto nitrites, then the other nitrite eating bacteria will eat that and in turn produce nitrates.
Your next thing after getting rid of the nem is to get an API saltwater testing kit, it will help you cycle your tank.
As far as the fish already still in it, it might or might not survive the cycle. In the olden days people would use fish to cycle their tanks and if it died it was oh well. These days most if not all people promote fishless cycling.
if you keep the fish you will have to monitor the tank daily and do many water changes to keep the ammonia levels at 0 or close to it.
here is a link you should look at
Cycle your salt tank