Absolutely brilliant co-op games: Pandemic and Forbidden Island.
The game mechanics are similar, but Pandemic is more complex and has a darker theme. Pandemic is a good full evening game for teens and older, Forbidden Island is faster and would work even with quite young children. They both default to a maximum of four players, but they can both be easily expanded to five or six.
Dixit is a fantastic and very original card game; Tsuro is a fun fast party game that can be understood in seconds.
All of those have been reviewed on Geek & Sundry’s Tabletop.
http://geekandsundry.com/shows/tabletop/
If you’re new to tabletop RPGs, I would strongly recommend avoiding D&D. It just isn’t a very good system, and it isn’t very newbie-friendly either. You’d probably be better off with FATE Core or something similar.
From left field, if you’ve any room for a two-player chesslike strategy game, give Khet 2.0 a try.
