I guess this would be like reinventing the wheel and wastage of time if you are doing it just to complete a project. if you are doing it for learning purpose then it is good.
well if it is the first case then you can find number of free ASP.NET text editors which can be embedded in your web forms.
Have a look at the following link
http://rte.codeplex.com/[
^]
Hope it will help
Regards
Ahsan Sarfraz
Dont forget to mark as answered if the info was useful.thanks