how to do unit testing

Have you ever heard of google? try this link:
http://en.wikipedia.org/wiki/Unit_testing[^]

Unit testing is not as simple as it seems...
You get various types of unit testing.
To mention a few:

DO-178B Level A -> Branch Coverage + Statement Coverage + Boolean Coverage (MCDC)
DO-178B Level B -> Branch Coverage + Statement Coverage
DO-178B Level D -> Statement Coverage Only

That is just from the DO-178B Standard
There are various standards around.

Requirements Testing is where you test your software requirements and afterwards check your coverage to see whether there are code in your software which doesn't link to a requirement. In the process testing whether the requirements do what they say they do.

A typical software requirement will read: "The function shall open the main form."
You wil typically set your input data (input parameters/or whatever), or setup your initial data to a closed main form, call the Unit Under Test (Function Under Test) and after the function executed, verify your input_data Vs you expected_data.

The field is wide and you get many different methods of testing.


Example:

/* input/initial data */
Form obj = new Form();
obj.setVisible(false); // set to opposite as what we're expecting

/* Call unit under test */
EnableMainForm(obj);

/* Test Case Checks */
assert(obj.getVisible(), true);

Say that EnableMainForm had only one requirement but was doing a whole bunch of other things aswell not documented/linked to a requirement, your code coverage would show just that and you could log a bug for that reason.

Have a look a this stackoverflow thread:

What is unit testing

And here's a really small example:

Let's say you wrote a function that sums two Int32:

Int32 Sum(Int32 int1, Int32 int2)
{
  return(int1 + int2);
}

You could unit test this function by creating a Unit Test project and create a function like this:

[Test]    
public void Sum_Test
{
  Int32 result = yournamespace.yourclass.Sum(2, 3);      

  //2+3 = 5. So you want to make sure that YOUR function does the job right, hence the assert == 5.
  Assert.AreEqual(result, 5);
}

Unit test projects can be executed from Visual Studio or tfs builds, and probably from command line.

Now, what you should know is the difference between Unit Tests and Integration Tests, because most of the people are doing the second ones believing they are doing Unit testing.

Unit Testing

Integration Testing

You also should have a look into Mocking.

Mock Object

Once you get into it, I'd really recommend having a look at Pex and Moles:

Pex and Moles

Hope it helps

Repeat this over and over again until it's the first thing you think of when you have a question like that.

"Google. Google. Google."