Where to start?
It depends on what you're comfortable with, and whatever urge(s) or idea(s) you already have. Also, if you have good ideas which won't really work together, don't be afraid to do multiple pieces. At the same time, probably not a good idea, so work with whichever one just feels more urgent, more "spicy".
Take me for instance. I've had, for awhile now, I decent fantasy plot worked out in my head, enough for a few novels in material. I have often (but not often enough) sat down to write plans for this thing, only to finish a few preliminary details and shove the bulk under a rock for later. I still plan to write it, but I keep having something else come up instead: a short story series about Death.
For the former fantasy series, I knew that I liked fantasy, and creating whole worlds from scratch, so I started there. I've used different magic systems, main characters, and plots in my head, but the world itself has stayed mostly the same. I jsut built it from what I liked, but I have to keep working.
The short stories , on the other hand, were much more spur of the moment, make it up as you go. If I get any of them published online, I'll try to send a link here, hopefully you won't hate 'em. The lowdown with these is that I challanged my Mom and Dad to a writing "contest," for lack of a better word. We both wanted to write, but neither was really going to. So I said "I'll write if you will." Thus began an installment of stories. (a thought just flew through my head of makiing a story about writing these stories). I had no idea really where to start, but then the Death concept just came to me, so I sat down, and started writing what felt good.
That's another thing: feeling good with it. I scrapped a lot of good ideas with my fantasy series because at first I liked them, but afterwords I hated them. This seems to happen to everybody, it's inevitable. Some ideas stay fresh in your mind and just don't fade, but others will. There isn't really a good way to tell if an idea will do that to you at the time you think of it, so the best thing to do is just write it while it's still good. If it's down on paper (or .doc), you're less likely to hate it later than if you haven't written it yet.
Originality. All stories are original (unless you have really really really REALLY bad luck and happen to think of exactly the same plot, characters, setting, and devices that another person does), but anymore, they all have overlapping elements and themes. If you're going to write, this is inevitable. The solution: write it
your way. Suppose you want to write a book about Orcs and Elves fighting each other. Deja vu...unless you write it in such a way that
you like and think of. Take Jim Butcher for example, either series. In DF, you have the magical hero who fights the baddies, meets girls, allies, and ahs his own personal demons to deal with. Again, deja vu. Then along comes the right and honorable Jim Butcher, and suddenly, Harry Dresden is born! From what I understand, the Dresden Files weren't a major project for him either, until they proved so popular and good. Possibly, the story you hate or don't think much of is your best work. For that, you need peer review.
