What is it that you're asking? How to self-publish a book for public consumption through iBook or merely how to format a document so that you can drag it into iBooks solely for your use?
If you are talking about selling a book through the iBookstore then you can only do that by getting a contract with one of the publishers who has a contract with Apple for the iBookstore. And since every book sold on the iBookstore is also a book that is sold as a print copy, basically you need to write a good enough book that a major publisher is willing to publish you.
Now, if you are simply asking how you can create something yourself that is readable using the iBooks application, then for now the answer is you need to write something and convert it to the ePub format, then open it with iTunes on your computer and get it in the iTunes library. Then the next time you plug in your iPad it will sync to your iPad and you can open it in iBooks. (This is presuming you have auto-sync for books turned on in your syncing preferences.)
Or, alternatively, you can wait until the update (announced today) to iBooks happens, when iBooks will be able to read pdf's. Then all you need to do is create a pdf file.
It IS possible to self-publish directly onto Apple's iBookstore... I have done it with my novel 'The Dare Ring', which is accessible through the iBooks app on all iPads and iPhones. I'm based in the UK, so it is possible here too.
It wasn't all that easy - creating the epub book is non trivial as none of the free conversion tools did exactly what was needed. But it can be done. Once you've created the epub document (and triple checked it - some of the conversion tools make unexpected changes) - then uploading it through iTunes Connect is relatively straightforward.
Forgive my enthusiasm, but if you like thrillers or Fight Club, check out The Dare Ring by Titus Powell. Only $2.49 and already getting lots of acclaim.
@Titus: Your book sounds interesting, as soon as I finish a couple I've been reading, I'll probably pick up "The Dare Ring". I'm a big Chuck Palahniuk fan (of his older stuff, less of his new stuff from the past few years).
You can go directly through iTunes Connect if you have your own ISBN and think you'll make a couple/few hundred dollars (that's about their minimum pay threshold). If not, they list several "aggregators" like Lulu or smashwords or a few others I think, that allow you to publish through them. Many supply ISBNs free or at little charge and pay out with even lower thresholds (ISBNs get really cheap if you buy them in large enough bulk, and with the aggregators, the pay threshold is much less of an issue since they have lots of authors).