in writing test cases we first perform the following steps.
first we need a test case
1.test case id

2.functinality

3.expected output

in writing test case we feel the negative affirmation how 
much we think negative abt the product the much negative we 
think better is the test case we write.

while writing test cases the first step is 
testcase id    
objective what r u testing
eg. to verify the functionality of login box accepting only 
alpha numeric char.
its procedure how you check
expected result
actual result.(means we comes to know testcase is pass or 
fail. when we input valid data it is passed or input 
invalid data such as special char or put it blank its fail)