These are the two TOR repositories:
deb http://deb.torproject.org/torproject.org <distribution> main
deb http://deb.torproject.org/torproject.org experimental-<distribution> main
where you substitute the name of your distro (Debian: etch, lenny or sid; Ubuntu: lucid, karmic, jaunty, intrepid or hardy) in place of <distribution>.
To add the repository key, use the following command:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 886DDD89
More info on using and configuring TOR: here and here.