Rather than me regurgitating what has already been written, open up "Books Online" (the Help system for SQL Server) and lookup "Missing Indexes". Make sure that you understand the warnings and limits of the related features, as well.
--Jeff Moden
RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row. First step towards the paradigm shift of writing Set Based code: ________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.
Change is inevitable... Change for the better is not.