How do I keep my fish alive?

You need to do your research. A saltwater tank needs to have a properly set up ecosystem in it, which can take 6-8 weeks to establish, before you can add any fish in it. You didn't give water parameter readings, live rock status, water temperature, and so on. Therefore we can't tell what's wrong but the fact that you didn't mention those probably indicates you don't know you should keep track of them.