You are not required to post your story/art to AO3, but we encourage doing so. It lets us have a central location for all of the Big Bang fanworks, and also, as you probably know, all stories are easily downloadable in e-reader friendly formats; this is particularly nice for your fellow fen who have health issues or who just plain love their e-readers.
The collection for the current year is Pros Big Bang 2013. (Previous years' collections are accessible here.)
A couple years ago I wrote up a little guide for how to post to AO3 for the Big Bang; the same directions apply, except that you'll want to make sure you're in the 2013 collection this year.
A couple of reminders:
1. In case anyone still doesn't have an AO3 account who wants one, let us know; we still have invite codes (or at least, I still have some). Alternatively, I have heard that the wait time for invites is very low, and if you apply for an invite you should get one within a few days.
2. If you have stories/art from previous years that you have not yet posted to the AO3, we have left the previous collections open for exactly this reason. Feel free to add your Big Bang story or art to the appropriate year's collection.
3. If you are planning to post in HTML rather than Rich Text, and (a) you are reasonably conversant with the features of your word processor and (b) that word processor is OpenOffice (it's free!), it may interest you to know that astolat has a set of OpenOffice macros for formatting italics/bold/etc and turning your story into HTML suitable for copying into a LJ or AO3 post. I can vouch that these things are real timesavers.
Happy posting! Let us know if you have any questions.