December 31 was two days ago, so it has already happened.
As for what you can do it depends on your precise requirement. If it's an app you made, or if you know who made it, you could investigate whether it could be updated to use another technology.
If its an app you individually want to keep using you can download it/archive it locally and use the standalone Flash Player.
If it's a corporate app which needs continued support Harman offers solutions for such, and you can contact them.
If it's an app on a public web site, perhaps a game you've developed, there's not much you can do as the people who play it are going to find Flash stops working in their browsers in a matter of days, if it hasn't already.
A couple of sites that I know of, Newgrounds and Kongregate, have tools to let players keep on playing Flash games. But I am not sure how well they will work, and I suspect most players will simply stop playing Flash games once Flash is removed from their browser.