Hi,
Have a look at design patterns on wikipedia or just google it. The Singleton and Factories are two types of design patterns I can think of off the top of my head but even a small game needs lots of different design patterns to work in unison to achieve a working program. Also have a look at the MSDN help files for XNA and how the XNA design team went about it I learned a lot from doing this plus you get loads of xna experience :)
XNA app development: http://msdn.microsoft.com/en-us/library/bb203873.aspx#ID2EVAAC
XNA Game Component Development: http://msdn.microsoft.com/en-us/library/bb199634.aspx
RSS of Shawn Hargreaves Blog: http://blogs.msdn.com/shawnhar/rss.xml
Good luck.
"I have more fingers in more pies than a leper at a bakery" -Gene Hunt. Life on Mars.