Allright, I'll start the list, although I am pretty new to this myself, and I am sure BigGourami will fill in where I left off, he is much better at this stuff than I am.
First decide what you want, such as fish only (FO) fish only with live rock (fowlr) or a reef tank, etc.
But in any tank:
-Ability to read stickies lol
-Patience
-RO water, or a unit to make such
-Salt
-Hydrometer or Refractometer to measure that level (I'd choose the latter)
-Marine Test kit
-Filteration of some sort, and there are quite a few options with this one, some of which don't even involve filters at all.
-And with regular water changes you can probably skip the skimmer, as it's a smaller tank
Most likely the setup you choose will also include:
Live Rock
Substrate, of either sand or crushed coral. I choose to not run live sand, as the rock is live, and the live sand will become live later as a result, so it's an overkill imo
And then depending if you do corals or not, you will need certain lights, and additives (and then test for those additives, but I recommend starting more basic at first)