A fresh Redmine installation has only one user account, which has administrator privileges. You can see it in the following screenshot:
This account is exactly the same by default on all Redmine installations. That's why it is extremely important to change its credentials immediately after you complete the installation, especially for Redmine instances that can be accessed publicly.
The administrator credentials can be changed on the Users page of the Administration menu. To do this, click on the admin link. You will see this screen:
In this form, you should specify a new password in the Password and Confirmation fields (actually, you should have done this in the previous chapter). Also, it's recommended that you change the login to something different. Additionally, consider specifying your email instead of [email protected] (at least), and changing the First name and Last name.