There's endless info out there and some of it can be false. My best advice is to research on your own, from a trusted source. AA articles can be a good start for basic start:
Important Information for Those New to the Aquarium Hobby - Aquarium Advice
Cycle your salt tank - Aquarium Advice
When I first started I checked out library books and looked on liveaquaria.com for livestock selection and some info.
The equipment depends on if you want corals or not. Either way, you don't need a skimmer since it's a small aquarium.
Other than what you have:
-1-2 lbs of rock, depending on what looks good and how porous the rock is (can have 75% base rock and 25% LR
-Filter rated up to twice as much as the water volume
-Test kit!!! API will work.
-light for FOWLR
(fish only w/ live rock) can be anything
for reef depends if you want soft or hard corals, or both and how much you're willing to spend b/c light fixtures + bulbs (or LEDs) can get expensive.