This depends on a number of variables.
Assuming you have the tank cycled properly (ammonia, nitrites and nitrates read zero), and you have the lighting required for soft or hard corals, you can safely add corals.
But, if you are new to the hobby, buy a soft coral like a toadstool coral or a rock with some mushrooms on it.
I would not add SPS hard corals unless you've put Metal Halides on the tank and it has matured.
Do you have a sump with calurpa? How much live rock do you have? These are important elements to have to keep your water quality up.