First thing you need to do is stop panicking. Fish have been reproducing successfully for several million years, far longer than humans and other mammals. This is not an emergency situation.
Second thing you need to do is to realise that you are human, and to stop projecting your mammalian instincts onto a non-mammal species. The platies don't care if the fry live or die. They produce loads of fry, (the 15 mentioned above is a minimum) every 4-6 weeks, on the premise of quantity, not quality. We mammals produce relatively small numbers, and take care of our offspring to ensure they survive. Platies DON'T. They assume that some will survive, and that is what has worked for them, and their forebears for millions of years. You should not care either.
It is possible to save large numbers of platy fry, by using the breeder boxes. However, unless you have room for dozens of tanks, or an outlet which will regularly take 3-4 dozen fish from you, you will soon have more fish in your tank than your filter can cope with, and your fish will then poison themselves with their own ammonia.
The photos that you have posted aren't particularly good quality, and it is impossible to say with 100% certainty that the fish is gravid. As has already been pointed out to you, a female platies are basically fry-factories, and will spend the majority of their lives being gravid. So I would say it is 98-99% certain the your fish is.
What should you do to prepare? Nothing. Let nature take its course, the chances are that some fry will survive on their own, but in small enough numbers that you and your tank/filter can cope.
