Be patient.

Trust me you will KNOW when your platys are pregnant & about to drop. They will look like little balloons about to pop.
When I got my first platys, I couldn't wait for them to have fry -- but then they started having fry on a (roughly) monthly basis. I was totally overrun LOL, so I've started giving all my livebearers away.
Anyway, if you go with a breeding trap -- definately buy the mesh-net type and not the solid plastic ones (babies can get caught in those).
Also, as Sasha mentioned, Hikari 'First Bites' are excellent. Or you could get Liquifry No.2 (though I've found it kind of messes up the water). I've also had platy fry do just fine on the regular flake food I feed the big ones as long as I crumble it up a little extra in my fingers before I put it in the tank.
How big is your tank? You might think about getting another female or two if you have room, or getting rid of one male ... the "best" ratio for livebearers is 1 male for every 2 females, otherwise the females can become exhausted from the males chasing them around & trying to breed constantly.
Anyway, congrats! Platys are so much fun.

Let us know how it goes!