Did you search the site :-)?
You might read this article: http://www.agileconnection.com/article/do-you-need-write-test-cases
You should read Lisa Crispin's and Janet Gregory's book, "Agile Testing", http://www.agileconnection.com/book/agile-testing-practical-guide-tester...
Consider reading Elisabeth Hendrickson's Exploratory Testing book, http://pragprog.com/book/ehxta/explore-it
Lisa, Janet, and Elisabeth are three of the best-known writers about test techniques. If you start with them, you are well on your way.