Good you to see your efforts, But how come before giving you programming lesson they assign you a project.
Nevertheless, As per my understanding you want to show everything in rich-text box whatever user clicks in the dropdown item in the combobox.
RichTextBox1.Text += "Your DropDownItem ";
It would be good if you show us your code :)
More precise.
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
richTextBox1.Text += comboBox1.SelectedItem.ToString();
}