In general; when you have algae growth. That's a general indicator that your tank is producing the nutrients and such that are needed to support reef life. This is just one indicator, however - testing the water for ammonia, nitrite, and then nitrate will be a more quantifiable guide.
Anything you add to your tank now stands a better chance of dying than surviving at this point.
You could (should, really) set up a quarantine tank (QT) anyways. If you were to fill that with rocks and water from another established tank, such as from your local fish store (LFS), that would give you a place to start while your display tank (DT) is cycling. Even that "newish" tank could experience a mini-cycle as it balances itself, however, so you would likely want to wait at least a week or so (guessing there - better to measure the same three chemicals above).
Once that tank was stable, you could - conceptually - start getting coral frags, inverts, and even fish. BIG caution here, however: you have no idea how long it will take your DT to cycle. So anything you put into the QT would have to be able to stay in there for - potentially - months (it shouldn't take that, but better safe than sorry, IMO). This includes things like pumps for flow, lights, filtration, heat, etc. There's also the caution that you don't yet have the experience with even an "empty" tank. The options - and time - you have to deal with problems greatly decreases when you add something alive into the tank - not to mention the stress level increases.
My suggestion is to wait. Cultivate patience now if you want to have a reef that grows. If you REALLY want something to do, look to one of the "cycle in bottle" products available. For example; I used the
Red Sea Reef Mature Pro kit to kick-start my tank. I don't think it really sped things up overly much and I may have had a perfectly fine "natural" cycle without it, but the kit did give me something guided to do nearly every day so that I didn't mess up my tank by "working" it too much.