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I would start with the online docs:
Oracle Database 2 Day + Performance Tuning Guide
http://docs.oracle.com/cd/E11882_01/server.112/e10822/toc.htm
Oracle Database Performance Tuning Guide
http://docs.oracle.com/cd/E11882_01/server.112/e41573/toc.htm
You also need to set tuning goals. Remember "faster" isn't a goal.
What is running slow?
Where are you going to focus your efforts?
etc...
If you are licensed for it the Performance Pack in Enterprise Manager has some very nice advisors that can help a lot.