What Torrent software should I use? 
Vuze and uTorrent are the most recommended and easiest to use:
Download Vuze here.
Download uTorrent here.


How to seed a Torrent?
In order to seed a Torrent you need to make sure the files you want to seed are in the right folder, if they are and you've got peers connected to the Torrent it will automatically start seeding if your settings are set by default.

Please make sure your PC is on and your Torrent client is active for seeding, once you switch it off the connection will be lost, if other users have not completed the download everyone will be waiting for you to reconnect, you should consider this when posting new Torrents as it's up to you to seed until others have finished.

P.S.
Please note, that some providers might block p-2-p connections and you will be unable to seed. To avoid such situations read the policy of your provider concerning torrents.