What was the diet of a stegosaurus?

That depends on the type of dinosaur. Most dinosaurs were herbivores, and ate whatever plants existed at their time and location, including conifers, cycads, ferns, seed ferns, horsetails, and more.

Small, carnivorous dinosaurs ate insects and small vertebrates. Larger ones ate small animals, and big carnivores ate large herbivorous dinosaurs, with the exception of Spinosaurus, which ate fish.