The most important thing you need to do is set up a page where people can report problems to you. Here's an example:
https://www.warzone.com/ReportBugThen fix the problems that are reported.
Sometimes you'll find users who experience problems but still don't use your page to report the problems, or tell you about it in any way. Don't worry about those users, as you need to find the ones that are good at communication.