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. 🙂