The mods deleted the old Newsgroup thread as people were posting links to sites so doubt we can go into much detail but in short you need three things:
Client: sabnzbd is the best and free
Access: Google for a provider, use one with a free trial to start
Search: Again Google for an nzb search site, plenty are free
That's all you need. You search for something on the site, eg Linux distribution, sabnzbd opens it and downloads it from your access provider. Bear in mind that if you really like Linux you'll want to pay for a good provider.