As a software tester, you should always think like a person who is using it, and you need to imagine various scenarios, which will happen in the real time environment. If you do this creative visualization you are rest assured that the application is more or less an error free. Just always see the application in a Negative way, as a software tester your attitude must be always negative with the application, if you have this attitude you can easily find out the mistakes.
The basic objective of writing test cases is to validate the testing coverage of the application. Well written test cases can make the testing cycle smooth and efficient. A good test case is easy to determine if a feature of an application is working correctly.
I have given some valid Test Cases for a Login credentials below