I think there's a site called applesnails.net? It's like this one, dedicated to all things apple-snail, haha.
From what I read myself over there, they'll eat most vegetables.. cucumber, romaine (not iceberg) lettuce, carrots, etc. Algae wafers and gel foods are also great if you can get them to eat them.
Remember to leave a couple inches of space from the top of the water to the top of the tank.. apple snails are amphibious and will crawl out of the water. (Also your tank, I hope you have a lid.)