First, you may want to review the answers in the section What do all these words mean? (seeding, uploading, share rating, etc.). A seed is a client which has a complete file. Seeding is the process of connecting to a torrent when you have a complete file. There are two ways to do this:

• ...by leaving your client open after the download completes. Once you have the entire file you become a seed, and the BitTorrent client remains connected to the swarm, sending to other users until you close it.
• ...by clicking on a torrent link (or opening a saved .torrent file) and selecting a filename of a file that has already completed. BitTorrent will check over the file and realize that it's already complete, and continue to connect to the tracker and serve as a seed.

It's generally considered a good idea to leave your client open as long as possible, since it helps other users. Some communities have guidelines on when it's permissible to disconnect, typically after the ratio of bytes received to bytes sent reaches 1:1, or 24 hours after the download completes. Please be nice, and do what you can to contribute to other users.