It depends on what you're coding for as to how you design your classes. Is there anything in particular that you're having trouble coming up with a design for?

For easily reusable/flexible code, strive for the most encapsulation you can. Encapsulation means the implementation is hidden, and only the interace is exposed. Keep all member data private and provide functions to work with that data. The more public/exposed/unencapsulated data you have, the more clients (as in you) rely on that data not to change, meaning they rely on its unflexibility. The more encapsulation you provide, the more opportunity you have to change implementation (faster algorithm, different container, etc.).

This applies to protected data as well as public. Any class that derives from a base class relies on the protected data as much as the public data. In other words, protected data provides as much encapsulation as public data - meaning essentially none. Keep all data private.

EDIT: and listen to Oluseyi.