1) Stop adding any livestock
2) You would need to allow a newly set up tank to cycle for at least 4 weeks
3) After 4 weeks you would need to test if the water is suitable for your marine livestock, you would need to test for ammonia, nitrite, nitrates, pH and salinity.
4) Ensure you have proper filtration system, using external filter with bio rings and charcoal can be a timebomb, use a sump with appropriate filters like skimmers.
5) Add in corals slowly.
6) If you want to add in fishes, you need to properly acclimatise them before adding them in. you can read more about acclimatising on this forum.
Like what comy shared, read through all the pinned articles on the forum, and post questions on the forum.