I started with a 20 gallon saltwater tank. It had a few dry rocks, sand, and 2 clownfish. Through having my clownfish get sick multiple times, I progressed to an 8 gallon nano. In there, I had my 2 clownfish, shrimp, gobies, etc. At that point, I knew a lot about the hobby.
Now I own a 75 gallon mixed reef, and it's been going great. Seriously though, you have to realize what you're getting into. I spent > $5000 on my tank so far. You also have to have patience... corals aren't going to populate your tank overnight. It takes some time.
I completely disagree with having an RO/DI unit to start. As it's not bad to have it, it's just going to complicate everything more. I don't have an RO/DI unit, I buy pre-mixed saltwater from my LFS, and top off with freshwater from my LFS (distilled.)
Anyways, all you really need to start are the following:
1. a fish tank (preferably 20-55 gallons to start)
2. live sand/live rock (to speed up the cycling process)
3. a heater (or a chiller, depending on where you live/fish you want to get)
4. Filtration (mechanical, biological, or chemical (research more in depth))
5. Lighting (LEDs, Halides, etc)
6. Sump (depending on size of aquarium, and plan)
7. Patience... and a lot of money.
'njoy the hobby.
