To add the rich text editor (default is FCKEditor in Community Edition) to your module, include the following reference at the top of your .ascx control:
<%@ Register TagPrefix="dnn" TagName="TextEditor" Src="~/controls/TextEditor.ascx"%>
then, in your .ascx control's markup, place the control as follows:
<dnn:texteditor id="teContent" width="100%" height="400" runat="server"></dnn:texteditor>
Then, in your codebehind you can access the properties of the text editor as follows:
To load content into the editor: teContent.Text = myTextString
To retrieve content after editing: myEditedTextString = teContent.Text
One good feature of open source code such as DotNetNuke is that you can examine the source code of the framework itself and of core and other free modules in the DotNetNuke Forge to see how techniques such as this can be done.