Step 1: Log in to Microsoft
When ordering the product, you received an email with a Microsoft 365 username (= Login ID) & password. You can use these credentials to log in to login.microsoftonline.com.
Step 2: Enter your username
Step 3: Change your password
Since this is the first time you are logging in, you need to change your password.
Once this is done, you are logged in to your Microsoft 365 environment for the first time.