Office 365 is a cloud service, which means it’s hosted “somewhere else” and this has one very significant advantage. It’s not dependant on anything you own or maintain.
So, as long as you have access to a web browser (Internet Explorer/Edge, Chrome or Firefox) you can log in to the service and use most of the functionality.
To do so, you need your Office365 credentials, which consist of a username and password.
Then go to the URL for the portal, which is https://portal.office.com and login.
Here is a Microsoft tutorial page on signing in.