This depends on who and what you are.
I'm an individual
OpenAthens is a tool used for institutional access. For example: universities, corporations, hospitals have site licences for journals and databases. These organisations use OpenAthens as the way to identify a user who wants access as one of theirs.
If you are a member of an organisation that uses OpenAthens, contact the library or similar part of that organisation. The "contact your organisation" function can help: https://login.openathens.net/auth#forgottenpassword.
I'm a librarian, information manager, IT department or similar
You've taken your first step into a larger world.