MySQL Question - How to handle multiple types of users - one table or multiple?

I'm designing a database for an organization that has multiple "types" of users. At first, I created only one user table. However, while all users share some common information (first name, last name, username, password, etc.), each user type requires one or two additional fields that are not applicable to all users. While I can create these additional fields and set them as NULL, I do not wish to do this, as the fields are foreign keys and its been causing problems for me.

How is this situation normally handled?

Thanks!