Tank, Salt Mix, RO/DI or distilled water, Heater, Powerhead(s), Filter or Live rock. Hydrometer or refractometer. That should be good for a Fish Only. Of course the typical stuff substrate (usually sand), Live Rock if you want a FOWLR. Now if you want a reef then you need more stuff and depends on a hundred different factors.
__________________ (5.5g)(36g Bow front community tank)(40breeder reef project)