Six things: the tank, a filter, a light, substrate, a heater if the room gets cold, and a water conditioner to remove chlorine. That's the whole starting list.
Buy the biggest tank your space and budget allow. A large tank is genuinely easier to keep than a small one — more water dilutes mistakes, so the temperature and water quality swing less.
Set it up and run it with no fish for a few weeks first. That step is the one beginners skip, and it's the one that decides whether the fish live.
See also