I have been using Bit Comet for a while now, but I wohaven’t seeded any files. How do I do this?
Just let your downloaded (completed) tasks upload for as long as possible, surpassing the minimum ratio of 1:1 
As soon as a downloading (leeching) task gets 100% of the torrent, it automatically changes to a seeder. In that sense, you are seeding.
If you mean creating your own torrents and seeding them, look here: How do I create a .torrent file? How do I seed it?