I can't figure out how to download a movie

When I try to rent a movie from iTunes on my iPhone 4s a sign pops up that says "not enough available local storage to download. You can manage your local device storage in settings" but I still can't figure out how to fix it. Does anyone know how what I can do to fix it?

You probably have to delete some stuff from your phone. You should be able to do that in iTunes, and sync your phone to free up memory.