Writing test cases is most important key activity performed by tester in software development life cycle(SDLC). This is the first step, followed by tester to identify, define & analyze the requirement.

Test Cases: A test case is a set of conditions or variables under which a tester will determine whether a system under test satisfies requirements or works correctly.
Test case have following elements :
* Test Case id: Unique Test cases identification Number.
* Test Summary: Objective of the test case.
* Test data: Which test data you are using while testing.
* Steps: Which steps you are followed while testing.
* Expected Result: Expected result of test scenario.
* Actual Result:The actual result of the test to be filled after executing the test.
* Status: Pass/Fail status, after executing the test scenario. Other statuses can be Not Executed if testing is not performed and Blocked if testing is blocked.
* Comment :Any additional comment on test cases.
* Created By:Name of the author, who created the test cases.
* Date Creation: Date creation of test cases.
* Executed By : Name of the person, who executed the test cases
* Date Of Execution: Date Execution of test cases.

Writing a Good test cases you have to remember following things:
* Your test cases should not be duplicate.
* Your test case should not be repeatable.
* All positive & negative test cases are covered.
* Write in simple , easy to understand and short summary.
*Test case id must be identify in your excel sheet or Management tool.
* Writing the test case steps should be in detail, accurate and clear.

If you are writing test cases in excel sheet then you can simply mark the status of test case. If the test case is failed then you should log a defect add same defect id in the test case while executing the test case. This will really help for tracking the testing progress & measure the quality of the application.

For more details, you can go through the following link.
How To Write a Good Test Cases