How to use Usenet newsgroup?

There is no "official" usenet.

Those are just usenet providers.

>How do I search usenet for something without paying for it?

Usenet search engines can be used for free.

But to actually download, you need to sign up to a Usenet provider. Once you are a paying member, go to any Usenet search engine and search for anything you wish to download. You then download a .nzb file (it's pretty similar to a .torrent file) which is opened using a Usenet client.

I prefer Unzbin, http://www.unzbin.com/ but you can choose what you like.

This is a tutorial to get you started:

http://www.gizmodo.com.au/2009/08/how-to-use-the-h...

The tutorial shows you how to use SabNZB, which IMO is a little harder to use. You can use UnzBin as an alternative.