Check with your publisher, if you used an outside company to publish your ebook. An ISBN might be included in your package, or available as an add-on.
Decide how many you need. If you have just one ebook, in one format, and don't plan to write any more titles, you can probably get by with a single ISBN. However, it may be cheaper to get a block of them, and you will need a separate ISBN for each version -- electronic, print, CD, etc.
Visit ISBN.org to purchase your ISBN. You may purchase single numbers or blocks of 10.
Confirm your assigned ISBN, and then copy the number and add it to the copyright page of your book. Be sure you have copied it correctly.
Add the ISBN to any physical versions of the book. You can add it by using a sticker or include it on the cover if the book hasn't been printed yet.