first step is to login to your Microsoft365 account. this can be done from here.
the above link will take you to the microsoft356 login page. login or create an account from here to proceed to the microsoft365 home page.
once in the home page, you will find the admin centre icon at the bottom of the list on the left. click on the icon to proceed to the admin centre. if the admin centre icon is not there then it means you do not have the permissions from your administrator to access it.