Sign in with the email address and password you set when the account was created.
Forgot the password? The login page has a reset link: enter your email, and a message with a reset link arrives if an account exists for it. The link opens a page where you set a new password.
Under Account, the Security page lists your active sessions - every device currently signed in, with its IP and last activity. If you see a session you do not recognise, press Revoke next to it and change your password immediately. Revoking a session signs that device out on its next click.
If two-factor authentication is enabled, signing in asks for a six-digit code after the password. Lost access to your authenticator app? Open a ticket from the email address on the account so support can verify you.