Hello! My name is Sandy.I'm reviewing your question now. Let's get started!
If you have forgotten the password ***** setup on the computer, there, is no way to use that password ***** the iPhone.
In this case, please reset your account password ***** the computer first and create a new password ***** then use that new password ***** the iPhone.
If you have any other queries regarding changing your account password, please let me know.
And if you can take a second to rate my service(-OK or more -by clicking stars at the top of the page and choose submit), that would be greatly appreciated.