How can be write Test Case ?

Purpose:-

Short sentence or two about the aspect of the system is being tested. If this gets too long, break the test case up or put more information into the feature descriptions. 

Prerequisite:-

Assumptions that must be met before the test case can be run. E.g., "logged in", "guest login allowed", "user test user exists".   

Test Data:-

List of variables and their possible values used in the test case. You can list specific values or describe value ranges. The test case should be performed once for each combination of values. These values are written in set notation, one per line. E.g.:

loginID = {Valid loginID, invalid loginID, valid email, invalid email, empty}

password = {valid, invalid, empty}

Steps:-

Steps to carry out the test. See step formatting rules below.

1. visit LoginPage
2. enter userID
3. enter password
4. click login
5. see the terms of use page
6. click agree radio button at page bottom
7. click submit button
8. see PersonalPage
9. verify that welcome message is correct username

Expected Results:- 

A results which you will get after run/execute of a test case.

Actual Results:-

 After run a test case manual or automatically  the result should be match with actual result if it is match with the actual result your test case goes in to pass status otherwise fail.