def start with a rocker, it will make your learning process much less painful. It may allow more bad habits during learning, however if you are serious about riding, you will turn out with riding technique that is just fine if you ride enough.
With around a size 12 (you will prob be smaller than that cause boots usualy are smaller than shoe size) i would prob go with a midwide board. something with around a 25cm waist width and above. But waist width is not the only thing to consider if you will have toe or heel drag. One must also consider your stance angles and width as well as the board sidecut and your bindings.
so i would start with finding the most comfy pair of boots and buying those, then, based on what size they are, get a board that will work for that boot size.
(boots are most important thing in snowboarding, don't skimp, just go with whatever pair is the most comfy)