Hey Liz :)
I'm not exactly sure what your tank set up is like, but you are going to have to make sure your tank is completely cycled and that you have hiding spots for the platies and for your betta. This could be anything from a cluster of silk or real plants to a bunch of decor and hidey caves.
From what I hear, platies are good tankmates for bettas, and most people don't seem to have issues. Of course, this all depends on the temperment of your betta and the tankmates. Some bettas can be very agressive, where as others not so much.
When you get your platies, rule number one is making sure they are all the same gender and that they did not come from a tank with sick fish and that they themselves are healthy. You're going to want to quarentine them for about a week or two weeks minimum to keep an eye out for illnesses which do not make themselves aparent in the first few days.
As such, you might want to buy a sterlite tubberware or something along those lines to house your platies during their qt. I know that 4 gallon sterlite tubberwares are all of 4 dollars at target, and they make wonderful hospital tanks should the need arise. If you don't have a spare heater to heat the tank, you'll want to to buy one or see if the tubberware will float in the tank to keep warm.
After the quarentine time, you'll want to remove your betta, move the decor around, add the platies (give them a day or so to adjust to the tank). During this time, your betta ought to be fine in the qt tank (clean it out first). The reason you are doing all this is because bettas are territorial and will act more agressively if they think the tank is stilll their territory and the platies are interlopers. By rearranging the tank after you remove your boy for the day, he probably won't recognize it as much.
After introducing your boy back in, monitor all the fishies' behaviors towards each other.