When you first start with a TNG family tree site, you were sent a temporary password by email. But that is not one of the passwords you usually use for your regular website work.
By the way, for Guild members, the Username is your Guild membership number, same as for logging into the Guild’s main website.
So go to your own TNG site, which you can find at: surname.one-name.net, replacing surname with your own surname of research. On the home page you will find a clickable link labelled “Login”. Click on that to get to the Login pop-up window:
Notice the check box labelled “Reset your password”. When you click there, a new line will appear for you, “New Password”. Fill it in with your preferred password and click the “Login” button.
Be sure to note your new password somewhere for future reference. If you do forget your password sometime, you can return to the login window and request that the password be emailed to you, or request a new temporary one.
Wiki article at: How to change your password when first starting