Finding the causes of poor performance in SQL Server

The best place to start for analysing and resolving performance issues are these two articles by Gail.
http://www.simple-talk.com/sql/performance/finding-the-causes-of-poor-performance-in-sql-server,-part-1/
http://www.simple-talk.com/sql/performance/finding-the-causes-of-poor-performance-in-sql-server,-part-2/

Go through them and u'll understand how to drill down to specific performance culprits.