How to find places in the code that should be covered by unit tests

I know that writing tests after writing code is not very good. I'm new to device testing and I feel that unit testing can bring many good benefits, so I'm obsessed with the idea of ​​reaching as much as possible.

For example, suppose we have this code:

public class ProjectsPresenter : IProjectsViewObserver
{
    private readonly IProjectsView _view;
    private readonly IProjectsRepository _repository;

    public ProjectsPresenter(IProjectsRepository repository, IProjectsView view)
    {
        _view = view;
        _repository = repository;
        Start();
    }

    public void Start()
    {
        _view.projects = _repository.FetchAll();
        _view.AttachPresenter(this);
    }

}

So, looking at the code above, you can answer me, what tests should I usually write on this code fragment above?

I run the write tests on the constructor to make sure that the FetchAll repository is being called and being called to a site like AttachPresenter.

WORK REPORT

Here is my interface:

public interface IProjectsView
{
    List<Project> projects { set; }
    Project project { set; }

    void AttachPresenter(IProjectsViewObserver projectsPresenter);
}

Here is a view:

public partial class ProjectsForm : DockContent, IProjectsView
{
    private IProjectsViewObserver _presenter;
    public ProjectsForm()
    {
        InitializeComponent();
    }

    public Project project
    {
        set
        {
            listBoxProjects.SelectedItem = value;
        }
    }

    public List<Project> projects
    {
        set
        {
            listBoxProjects.Items.Clear();   
            if ((value != null) && (value.Count() > 0))
                listBoxProjects.Items.AddRange(value.ToArray());
        }
    }

    public void AttachPresenter(IProjectsViewObserver projectsPresenter)
    {
        if (projectsPresenter == null)
            throw new ArgumentNullException("projectsPresenter");

        _presenter = projectsPresenter;
    }

    private void listBoxProjects_SelectedValueChanged(object sender, EventArgs e)
    {
        if (_presenter != null)
            _presenter.SelectedProjectChanged((Project)listBoxProjects.SelectedItem);
    }
}

WORK REPORT # 2

This is how I test interaction with the repository. Everything is fine?

    [Test]
    public void ProjectsPresenter_RegularProjectsProcessing_ViewProjectsAreSetCorrectly()
    {
        // Arrange
        MockRepository mocks = new MockRepository();
        var view = mocks.StrictMock<IProjectsView>();
        var repository = mocks.StrictMock<IProjectsRepository>();
        List<Project> projList = new List<Project> {
            new Project { ID = 1, Name = "test1", CreateTimestamp = DateTime.Now },
            new Project { ID = 2, Name = "test2", CreateTimestamp = DateTime.Now }
        };
        Expect.Call(repository.FetchAll()).Return(projList);
        Expect.Call(view.projects = projList);
        Expect.Call(delegate { view.AttachPresenter(null); }).IgnoreArguments();
        mocks.ReplayAll();
        // Act
        ProjectsPresenter presenter = new ProjectsPresenter(repository, view);
        // Assert
        mocks.VerifyAll();            
    }