Really, you started with a very simple subject matter, in fact one of the simplest I've ever SEEN in terms of basic shapes, and you don't know where to start?
Pixelarts should be treated like traditional art. For now, since you've done effectively nothing worth noting, I suggest you try. Start where you think is best to start, make some real progress. Post it, we'll crit, and then you learn a bit more. Rinse-repeat until you're personally satisfied that you can do no better.
Uh. It's a ball. A big ball. ... I think you should start... DOING something instead of showin' us a lineart of a ball. A ball that may or may not be either a trace of an MS Paint circle.
@Zee: I meant what should I start on? The markings? the Shading?
@xTyvonx: I been spriting ZX style for 2 years now, it's not that hard.
@Geo: What size would you suggest would be apropriate?
I've never played Okami, so I honestly wouldn't know. I'd think it can't be any bigger than the final bosses of ZX and ZXA though. Maybe around Bifrost's size?