Can anyone please suggest me, where I can edit the HTML Layouts of the site, I mean, like we do in wordpress like, there is a main index.php file, where we can add functions in the header, or editing the existed functions, and Where to go for single topic layout editing, like we do in wordpress by editing the single-post.php..
Many Thanks in Advance

You can try this:

default\themes\default\templates\forum\topic.tpl

Thanks Man, Finally Got it..