There are several different ways in which you can find directory information about a person. Within the Blitz system if you use the global address list - available via the Address Book - you can find their name, phone, Hinman box and department or class - amongst other information. Go to your address book and enter their first name in the Search field then press Enter.
There is also a website where you can find directory information. The Dartmouth Name Directory (DND) look up information is available to you at http://dndlookup.dartmouth.edu/.
If you need help learning your NetID, or finding the name associated with someone else's NetID, go to http://www.dartmouth.edu/comp/systems/netid.html.