In a testcase , there are two conditions PreCondition and PostCondition to be satisfied before executing a test case.

I need some clarification regarding the data to be entered in both of these sections. Is it some text which should write in both the sections.

Test cases can be run without either Pre- or Post-conditions. These fields exist to allow you the opportunity to record free form text information to the tester if such conditions exist. For example, preconditions might be that the user has to authenticate through a firewall using an external application to reach the proper server or that existing copies of file foo.bar should be removed from the test machine. Postconditions might relate to cleaning up the test machine once the test case is complete. Such conditions may or may not exist relative to each test.

The short answer is that it's a free form text field used to provide this information to the tester.

Can you define a pre-condition that forces a test case to be executed before another test case?  In other words, introduce some order into running the test cases?