Converting Wood Deck Porch To Concrete

I Have an older house that had a coal bin in the front. I would like to still keep that as a porch but I would like to seal it well. I was thinking that I could take off the wood porch and replace it with a concrete slab for a porch and that would seal it well but I have no idea how to do this. Can I purchase a pre-made concrete slab and install it or should I pour it and how much support do I need if any. The porch is about 4' wide and maybe 12' long.