All sites have an option Forgot my password. Just go to login page and click Forgot your password or username? It is important to remember the email address that you signed up with because the login details will be sent to that email. If you filled in all the details correctly and there's still no email with a password, check your spam folder.
Some sites offer to restore a password with a phone number that you filled in while signing up for a service so you can receive the login details to your phone.
If both ways are unavailable it's better to contact directly the support team of the site.
We may help you to resolve the issue. Just send us the name of the site and registration email to firstname.lastname@example.org.