How can I create and save a RTF file

One way to do this is to use the RichTextBox.SaveFile method.

private void button1_Click(object sender, System.EventArgs e)
{
    // Create a SaveFileDialog & initialize the RTF extension 
    SaveFileDialog saveFile1 = new SaveFileDialog();
    saveFile1.DefaultExt = '*.rtf';
    saveFile1.Filter = 'RTF Files|*.rtf';

    // get a file name from the user 
    if (saveFile1.ShowDialog() == DialogResult.OK)
    {
        // Save the RTF contents of the RichTextBox control that 
        // was dragged onto the Window Form and populated somehow 
        richTextBox1.SaveFile(saveFile1.FileName,
            RichTextBoxStreamType.RichText); //use to save RTF tags in file 
        //RichTextBoxStreamType.PlainText);//use to save plain text in file 
    }
}