Replacing Small Cracked Porch/Slab (with pictures)
For a slab that small you never really need to use rebar. If I were pouring a slab like that, I would make it at least 4-5 inches deep.
As mentioned by joecaption, the correct prepwork was probably not done. Check out YouTube... there are a lot of very good howtos on pretty much anything you will ever want to do. Here is one that may be helpful: