Reverse osmosis removes teh dissolved solids out of your tap water. They are not essential, i know reefers here in chch who don't use RO water for their reef tanks. My spare tank in the garage which has mainly soft corals and fish just has plain tap water, but i purify my water for my main tank which is where i keep my more sensitive corals.
In terms of corals you would probably want to start out with some easier ones like some leathers and mushrooms and once they are growing well then look at branching out to hard corals SPS and LPS. These are more sensitive to water quality and require a bit more care (eg monitoring your mag, calcium and kH levels and dosing additives as required).
In terms of the beginning your setup you'll either need to buy some dead rock and cure it which usually takes around 4 - 6 weeks, or you'll need to find someone selling live rock and use that. Benefit with live rock is you can start adding small fish etc immediately as the bacteria are still live. You still need to take care that you don't stock too quickly as there is likely to be some dieoff of bacteria during the time you get it and the time you put it in the tank.
With regards to fish, you need to take care with what you select and the order you put the fish in. Some of the small hardy fish that beginners often choose aren't actually the best fish to go with (ie damsels, tomato clowns) as they can become the dominant fish in the tank. Small percula or ocellaris clowns are pretty good starter fish and they don't need an anenome to live in, in fact i wouldn't recommend getting one for at least 6 months to a year after your tank is setup