When it comes to signing up for a PlayStation 3 account, you have several options when it comes to the username, or PS3 Online ID. You have the ability to use up to 16 alphanumeric characters, along with hyphens and underscores. If the PS3 ID you want to use is already taken, use the underscore character to insert a space. The underscore will show up just like a space in your PS3 ID and will make it unique.
Go to the "PlayStation Network" menu on your PS3 and choose the "Sign Up for PlayStation Network" option.
Enter your account information, such as account type, personal information, e-mail address, password and security question. After you have entered all this, you will reach the "Online ID" page.
Use the onscreen keyboard and your PS3 controller to select the alphanumeric characters you want in your PS3 ID and then select the underscore (_) character on the keyboard to put a space in it.