Add a Page where you want to Implement RichTextbox and Design the Page.
Now Run the Page. Look, how it looked.
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" ValidateRequest="false" %>
Now, you can save the Data in Database, Do postback, retrieve same in Rich Text box and do anything you want.
You can find many more articles written by me on Dotnetspider also. Have a look at them.
You can add me to your Circle on Google+ at +John Bhatt
Hope this will be helpful to you.