How do I "Close" one form and show another ?

Well, generally, you create and open one form from another and close the first one as follows:

formMain mf = new formMain();
mf.Show();
this.Close();

However, notice that if formLogin is your start up form (i.e. the form which is specified in Application.Run in the Main() method of program.cs), then closing it will cause the entire application to close.

If that's the case, you'll either have to hide formLogin instead or make some other form the start up form.