Northwestern allows you to access your email account through any web browser on a computer using the Outlook Web App, or webmail. Currently, there are two different forms of the Outlook Web App and the one that you can access is determined by whether or not your department has migrated its email to Office 365. You should have received communication if your email account was migrated.
If you are not sure if your account has been migrated, ask your department's IT support. If they are not sure, you can try to log into each of these portals to see which one allows you to log in. If you are still having trouble, contact the Northwestern IT Support Center at (847) 491-4357 or servicedesk@northwestern.edu.