What did Shelob Eat?....

[Laurinquë]

Shelob, if I remember correctly, ate a lot of prisoners Sauron didn't want anymore, and pretty much anything else she could find, mostly orcs.

[Nerwen]

Orcs and prisoners are indeed Shelob's diet:

And Orcs, they were useful slaves, but he had them in plenty. If now and then Shelob caught them to stay her appetite, she was welcome: he could spare them. And sometimes as a man may cast a dainty to his cat (his cat he calls her, but she owns him not) Sauron would send her prisoners that he had no better uses for

–"Shelobs Lair", The Two Towers.

I can't remember if the film mentioned what Shelob usually ate.