Some dinosaurs ate lizards, turtles, eggs, or early mammals. Some hunted other dinosaurs or scavenged dead animals. Most, however, ate plants (but not grass, which hadn't evolved yet).
The largest complete dinosaur we know of was Brachiosaurus ('arm lizard'); it reached 23 m in length and 12 m in height (about the length of two large school buses and the height of a four-story building).
Dinosaurs went extinct about 65 million years ago (at the end of the Cretaceous Period), after living on Earth for about 165 million years.
The oldest dinosaur types are known from rocks in Argentina and Brazil and are about 230 million years old. The most primitive of these types, Eoraptor, was a small meat-eating dinosaur.
Your local museums, public libraries, and bookstores are good places to start.
Paleontologists now have evidence that dinosaurs lived on all of the continents.
Paleontologists don't know for certain, but perhaps a large body size protected them from most predators, helped to regulate internal body temperature, or let them reach new sources of food (some probably browsed treetops, as giraffes do today).
There are dozens of theories to explain a probable cause or causes. Throughout the Mesozoic Era, individual dinosaur species were evolving and becoming extinct for various reasons.