First of all, this question has nothing to do ASP.Net or MVC for that matter.
And before I go on to answer this question, may I ask "What have you tried so far?". This is quite basic construction and instead of asking on forum like CodeProject, you better discuss with your colleagues or even your boss. That way not only you would be in a better position to discuss this quicker but your colleagues can give you input much faster than you would get on this forum.
Anyway, if I understand correctly you want to maintain a chain of conversion in the forum.
You can start with a very simple structure like this:
Posts:
Id,
Title,
Content
Replies:
Id,
PostId, //Id column in the Posts table
ReplyTo, //Id column in the Reply table. If this is the direct reply to Post, it remains null.
Content
Hope this help to start.