You should be able to use any internet domain name available in the standard public internet root servers. In other words, any regular domain name that ends with any of the standard top-level names such as:
or any domain name that ends with any of the 200+ standard two-character country code endings such as .au, .br, .ca, .cn, .de, .es, .eu, .fr, .mx, .uk, .us, etc.
Initially, each Bongo4U website is assigned a unique "system code." You may initially reference your website using this system code + ".bongo4u.com" as its URL. For example, if your website was assigned system code "widgets", then you could use URL http://widgets.bongo4u.com to access your website. The unique system code cannot be changed.
Eventually, your own domain name will probably be setup to point to your website so that the above "*.bongo4u.com" name won't need to be used to access your website (although it will be available). Refer to the "How do I setup my domain name to point to my Bongo4U website?" FAQ entry for more information.