Hi there. I have no experience with Torrent. How to find a file and then download it? eg. I want to find mp3 which start like that "50 cent feat. moby"

You need to find a torrent (a file ending in .torrent) file, in the search tab there are several websites you can search (isohunt, piratebay, ...). Once you have found one, click on the torrent and KTorrent will download it.

Here is an example torrent file :

http://www.slackware.com/torrents/slackware-10.2-install-d1.torrent