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.