Reddit reviews Marine Aquarium Handbook: Beginner to Breeder

Stop. Please do some research before diving into this.

This is a step by step book. It will help.
https://www.amazon.com/New-Marine-Aquarium-Michael-Paletta/dp/1890087521/ref=mp_s_a_1_2?keywords=michael+paletta&qid=1569031273&s=gateway&sr=8-2

I also recommend Martin A. Moe’s Marine Aquarium handbook. https://www.amazon.com/Marine-Aquarium-Handbook-Beginner-Breeder/dp/0982026218

Doing some research will be a lot better than me telling you that you are going at it wrong.

I also recommend keeping fresh water for at least a year beforehand to get use to the little things like testing, feeding, and water changes.

These are all great tips on how to get started... but the tips alone here arent enough to run a successful tank. This book is a relatively quick read and serves well as a reference for owning a tank. The important stuff is at the beginning, so you can read it and make purchases accordingly, then continue reading as your tank grows. It's not horribly difficult, and you can talk to your petstore owner about what animals are the most forgiving to newbie mistakes. Clownfish take some serious effort to kill, and so do many coral such as mushroom coral.

I kept a 29-gallon saltwater tank for several years; it was large enough to work.

The key with saltwater tanks is good bio; I invested in a big wet-dry filter (kinda similar to this but not quite as large) and plenty of good lighting.

Saltwater tanks are very cool, but they definitely require a lot more work than freshwater. Be prepared for significant time investment testing Ph, nitrates, etc. Also, buy a good book, such as Moe.