Unit Testing a Login in ASP.NET

I'm very new to TDD and I'm having trouble with one of my unit tests. I just can't seem to understand what to do next. :( I'm trying to unit test a service of mine called AccountService and I'm testing a method called DoLogin(username, password). Here's some example code:

    [Test]
    public void User_With_Correct_Username_And_Pass_Should_Login_Successfully()
    {
        // Arrange
        var accountService = new AccountService();

        // Act
        bool result = accountService.DoLogin("test", "test");

        // Assert
        Assert.IsTrue(result);
    }

    public class AccountService : IAccountService
    {
      public bool DoLogin(string username, string password)
      {
        if (string.IsNullOrEmpty(username) || string.IsNullOrEmpty(password))
            return false;

        return true;
      }
    }

So this test pasts but now what do I do?! How do I really test if a valid login occured? Do I need to implement an integration test and test the login against a real or in-memory DB? Sorry if I'm doing things totally incorrect. I really hope one day to get this TDD stuff down. Thanks