How to normalize a table with common and non-common fields? and thanks in advance.

I am trying to normalize a table "People", where each entry (person) belong to one of many different categories (e.g. "lawyer", "client", "judge"). Some fields are common across categories (e.g., firstName, lastName).

However, some (set of) fields are unique to each category: for instance, a "client" will likely not have a "LawyerBoardID" number. Therefore, an entry will have a value in that field if and only if the person belongs to the "lawyer" category; in other words, that field will be NULL for all other categories.

This has implications for child tables. One person, many entries in the "Address" table - textbook example. If I split my "People" table in many tables - namely, one per category - then I will have to put as many FK in the "Address" table, right? Therefore, if I have 10 categories, there will be 9 NULL FKs.

Any suggestion? Links to relevant web pages are also very welcome.
Your case looks like an instance of class/subclass (or, if you prefer, type/subtype).  You can find many answers like this one in SO or here in DBA.