How to write testcases ?

In Agile, writing test cases can be little different as compared to other methodologies. We never get entire complete functionality to test because we build the functionalities in iterations and only part of the functionality is delivered in each Sprint (aka iteration). So now whenever any Story is delivered in any Sprint. First thing a Tester must do is request the Programmer for a short Knowledge-Transition (K.T) session regarding this story. And we should cover the following points during the discussion: 1- What should be tested? 2- What should NOT be tested? 3- Any high risk areas? 4- Take