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:

If you are not a member of a participating organisation, you cannot get an OpenAthens account. You will need to use other means to access the content you need. The publisher will usually have a way for you to exchange money for access. 

I'm a librarian, information manager, IT department or similar

You've taken your first step into a larger world.

These pages will give you a flavour of how you would use OpenAthens. The best place to start to find out more though will be