I have a 20 and will soon have a 40 haha so I can help some. Do you want a reef tank or just a fowlr
to start? Basically you will need:
Salt: Instant ocean is cheap an works. If a reef use their reef crystals version.
Live/base rock: 1- 1.5 lbs per gallon
Sand: aragonite based, 1 lb per gallon
: you can buy one or get water from your lfs
. Buying one is cheaper long term.
Powerheads: hydor koralias are great beginner powerheads and do a good job. You want 20-30x total tank turnover per hour.
Lighting: for a reef tank I'd go with an eBay 120 watt dimmable fixture. If not a reef any light works.
API test kits are fine for ammonia/nitrite/nitrate and pH but for corals you will want a good test kit for mag/calcium and alkalinity. Hmm what else...idk lol. Hope this helps somewhat