It is really up to you, but I started with a fish only, because I did want some fish that weren't reef safe. Also because keeping a reef can be harder, not to mention a little more expensive, because you do need to buy better lighting. Reefs aren't as forgiving in water quality changes. Obviously you don't want to have bad water quality, but in the begining you may make small mistakes, that fish will foregive you for, corals won't.