Go to the login page at https://my.webinarninja.com/login and you'll see this page:
Enter the email address associated with your WebinarNinja account, as well as the password you created when you signed up, then click Log In.
Click Forgot Password underneath the password field to reset it. See this guide to learn more.
A Sign Up option is available on this page for those who are invited to co-host a webinar and for those who want to be able to see all the WebinarNinja webinars they registered for in one place. This is a free user account and can be upgraded to a creator account if the user wants to create webinars.
If you are a grandfathered user from the earlier version of WebinarNinja and are trying to access your account on the current version, see this guide.