Hello Gabriela,
The initial Administrator account used to login and create / manage other user account profiles is normally created during the last steps of the installation process, via the web installer. See for example:
If you don't recall the credentials you used to create the administrator's account during installation, you can try creating a new Admin account from the command-line, so you can log in with it and check the settings on the current account, change the password if needed, etc. To create a new Admin account from the command-line:
AtoM does not currently have the ability for users to register and create new accounts via the web interface - at the moment, a logged in Administrator must create additional accounts and assign their permissions. I have seen community-created plugins for AtoM that handle new user registration, but as far as I'm currently aware, they aren't all publicly accessible, well-documented, and maintained against recent versions.
If you do remember your credentials, and the issue is that whenever you try to log in, it returns you to the login page without logging you in, please see the following thread for suggestions:
Cheers,