Know what you're looking for. Before 2007, ISBNs were always a ten-digit number (although the last number is sometimes "X"). Since 2007, book publishers have begun using 13-digit ISBN-13s. Some publishers still use the older ISBN-10s, and many list both an ISBN-13 and an ISBN-10. ISBN-13s typically start with "978." American ISBN-10s typically start with "0" or "1" and are usually hyphenated after the first, fifth and ninth digits (ex. 0-7432-3601-7).
Look for the barcode. Typically, bar codes will be in the lower left hand corner of the back of a book's cover or dust jacket. For most books, the ISBN will appear above the barcode, and will usually be clearly labeled ISBN, ISBN-10 or ISBN-13.
Look on the side of the book. Paperbacks will sometimes have the ISBN on the bottom of the spine.
Look for the copyright page in the first few pages of the book. On the same page that has the book's copyright date and other publishing information, the ISBN is usually at the bottom. Sometimes copyright pages will list multiple ISBNs for different editions of the book, but the different bindings are usually noted in parentheses (ex. "Pbk." would identify the paperback version).
Search online booksellers by your book's title. Most popular websites like Amazon will show a book's ISBN on the product page. This only works though if the book you search for has only one edition/binding. Even if the site's image matches your book, you may not have the right ISBN; sites often use one stock cover image for multiple editions and bindings.
Search online databases. Sites like isbndb.com will allow you to search by title and author to get a book's ISBN. You can also use a book's copyright date to make sure you have the right ISBN for the edition you are looking for.