If you want to download something via torrent/bittorrent on ubuntu, you can use program called transmission. And the good things is you don't need to install tranmission because it comes pre-installed with ubuntu.
So you can use transmission right away, no need to install, you can search the keyword 'transmission' on ubuntu unity dash search.
To download torrent with transmission, all you have to do is get the .torrent file from somewhere and then open it with transmission and the download will be started.
~ happy download ~