I would write a test for anything that you are writing code for that is testable outside of the GUI interface.
Typically, any logic that I write that has any business logic I place inside another tier or business logic layer.
Then writing tests for anything that does something is easy to do.
First pass, write a unit test for each public method in your "Business Logic Layer".
If I had a class like this:
public class AccountService
{
public void DebitAccount(int accountNumber, double amount)
{
}
public void CreditAccount(int accountNumber, double amount)
{
}
public void CloseAccount(int accountNumber)
{
}
}
The first thing I would do before I wrote any code knowing that I had these actions to perform would be to start writing unit tests.
[TestFixture]
public class AccountServiceTests
{
[Test]
public void DebitAccountTest()
{
}
[Test]
public void CreditAccountTest()
{
}
[Test]
public void CloseAccountTest()
{
}
}
Write your tests to validate the code you've written to do something. If you iterating over a collection of things, and changing something about each of them, write a test that does the same thing and Assert that actually happened.
There's a lot of other approaches you can take, namely Behavoir Driven Development (BDD), that's more involved and not a great place to start with your unit testing skills.
So, the moral of the story is, test anything that does anything you might be worried about, keep the unit tests testing specific things that are small in size, a lot of tests are good.
Keep your business logic outside of the User Interface layer so that you can easily write tests for them, and you'll be good.
I recommend TestDriven.Net or ReSharper as both easily integrate into Visual Studio.
