After I add tracker's URL in Tixati's Trackers setting page, what else do I need to do?

Are you attempting to make a torrent or add trackers to an existing torrent.  If the latter, randomly adding trackers is unlikely to help.  If you enable DHT, some peers may eventually be found, but it's possible that no one is seeding the torrent, in which case, you're not going to get it.

Trackerless torrents are legitimate and functional, but do rely on DHT or PEX.

Trackers on the Options Trackers page are added only to opened torrents, not created ones. In order to add trackers to created torrent you should go to properties, trackers tab and right click > edit trackers list there, then save torrent file.