Start with this two-part series. Finding the problem is more than half the battle.

http://www.simple-talk.com/sql/performance/finding-the-causes-of-poor-performance-in-sql-server,-part-1/

I doubt there's a single, comprehensive site that covers everything. The links given earlier are good but that's far from everything. Read blogs, articles and performance questions here. Grant Fritchey's book (link in his sig) is a very good resource.