how to create a log file in vb.net

let me explain I have a window form in vb.net, User can add new customer using this form, but there is some type of check from the database while doing this, only if the requirements are met the user can add a customer, I already have a code to display error message box while doing this, I also want to create a log file with these messages, How do I do this?

If mylog.FileExists("C:\mylog.log") = False Then
       Set createlog = mylog.CreateTextFile("C:\mylog.log", False)
   Else
       Set createlog = mylog.OpenTextFile("C:\mylog.log", ForAppending, False, TristateUseDefault)
   End If
End Sub