In the visual editor, look in the dialogs section in the toolbox for the Open File Dialog. Add it to your form and call it from one of the button's click events or a menu's click event. You can then edit the properties of the dialog to tell it what type of files to display.

As for the extension, you have to get the path of the file from the File object returned from the dialog, and do:

extension = path_string.SubString(path_string.lastIndexOf(".txt"));

That should answer your questions.

I would give you more specific terminology, but I've only had web development on the brain for the past two or three months so I'd need a refresher on C# first.

1. It's one of the properties of the dialog after the dialog is complete:

if (myFileDialog.ShowDialog() != DialogResult.Cancel)
{
myFileDialog.Filter = "Text Files (*.txt)|*.txt";
myFileDialog.Title = "Open Text File";
string fileName = myFileDialog.FileName;
}

2. To draw a square, first you include the GDI+ drawing libraries (System.Drawing, System.Drawing.Drawing2D, etc.), then in the visual editor, edit the OnPaint event and add code. It goes something like this:

private void OnPaint(object sender, PaintEventArgs e)
{
SolidBrush brush = new SolidBrush(Color.Red);
e.Graphics.DrawRectangle(brush, 33, 33, 99, 99);
}