Bold or italic charecter in RichTextBox or RTF

I have a RichTextBox. I'm assigning the Text like;
....
          myRichTextBox.Text =
                    "Welcome," + Session["Name"] + Environment.NewLine
                    + "Your Age is " + Session["Age"];
....
Everything is ok. The Text shown as;

Welcome, Kosturdur
Your Age is 30

But how can I make the words "Kosturdur" and "30" bold or colored while assigning the value. I know that it's impossibe to do it with Text property so I have to use RTF property of RichTextBox
(RichTextBox.RTF =....).
The problem is I don't know can I do it. I tried but couldn't successed.

Thanks,