Hi Nishad,

Please check below blog. I hope that will help u
http://amitsalesforce.blogspot.in/2015/06/salesforce-testing-best-practice.html

http://amitsalesforce.blogspot.in/2015/06/best-practice-for-test-classes-sample.html
Unit Test Mechanism in Salesforce

1) Single action
Test to verify that a single record produces the correct, expected result. So created the unit test data in your test class.

2) Bulk actions
Any Apex code, whether a trigger, a class or an extension, may be invoked for 1 to 200 records. You must test not only the single record case, but the bulk cases as well.

3) Restricted user
Test whether a user with restricted access to the sObjects used in your code sees the expected behavior. That is, whether they can run the code or receive error messages. Always use RunAs for to check user access on sObjects.

4) Positive behaviour
Test to verify that the expected behaviour occurs through every expected permutation, that is, that the user filled out everything correctly and did not go past the limits.

5) Negative behaviour
There are likely limits to your applications, such as not being able to add a future date, not being able to specify a negative amount, and so on. You must test for the negative case and verify that the error messages are correctly produced as well as for the positive, within the limits cases.

Please let us know if this will help you.

Thanks,
Amit Chaudhary