Replacing highlighted text

Hello All,

I'm new to .NET. I have a form with a multiple line textbox. I want to highlight specific text within the textbox and then change the text by clicking on a button. Similar to how FrontPage works with text. In FrontPage you can highlight text and then click the bold, italic or underline, which then makes the hightlighted text bold, italic or underlined. I really don't even know where to start.

Thanks in advance for any help you may be able to give me,