I want only registered user allow to post a new topic and guest only can reply the topic. Please tell me how to do this?

I dont want guest can post new topic

Check: How to ban guests from new topic creation?

I have gone through this link but not able to understand where exactly need to change. Please help me on this.

Thanks.

Do I need to change in bb_specials.php as:-

$regUsrForums=array(3);?

What does it mean and how its working? I gone through forum but not understand how exactly it working. :)

Here are links which may help you. Forums search tool is your friend ;-)