Understand ISBNs: An International Standard Book Number is the unique code assigned to every book published, much like a UPC. A company called Bowker issues and manages ISBNs, and they aren't cheap: $125 for a single number, or $250 for a ten-pack. (I bought a ten-pack in 2005, and have used only three of them to date.) ISBNs are mandatory for all printed matter, but by and large they're optional for ebooks, as most ebooksellers don't require them.
The only real reason you'd need one is that Apple insists on it. You can obtain one directly from Bowker or arrange for one through an aggregator (such as Smashwords), which usually doesn't charge anything for the number but ties you to its service pretty much forever.
Set up a publisher website: It's good advice to create a publishing "company," or at least an author's website where you can link to all the places where your ebook is on sale. Most sellers link back to any available publisher site, and this helps with search rankings. My publishing company website isn't much to look at, but it covers the basics. A blog system would work fine.
Have a bank account ready: Most payments for book sales are wired directly to a bank account, or sometimes to PayPal. Have these account numbers ready to go when you're preparing to sell. You'll also need to provide your Social Security number or Employer ID Number to sellers for tax purposes.
Set the magic price: Pricing isn't too tough with ebooks. Unless you are publishing a highly technical textbook, you'll almost always have to price it at $9.99. This is the accepted and nearly required price level for all ebooks today: You'll make far less in commissions from most sellers if you go higher--and if you price it lower, your book will look cheap.
However, if your book is short--under 45,000 words or so--lower pricing may be required. You'll also find that some services have clauses ensuring that you don't charge less for the book somewhere else, which pretty much locks you in to one price for every platform.