the reply/post options for forums are part of the forum functionality itself, so if you place a Forums widget on the page it should show those options automatically. This does depend on the permissions settings (user probably has to be logged in to make a post).
There are other settings for the Forums widget that show different forums, groups or threads, which is outlined here: http://www.sitefinity.com/documentation/gettingstarted/configuring-the-forums-widget
but however you configure it the functionality should be there as part of the widget itself.
I hope this is helpful!