One of the first Dinosaurs was the Coelophysis. It was a smaller 3 foot tall Dino and was very fast. They were cannibals too. Fossil evidence shows an adult Coelophysis with a juvenile in its belly. They had nesting sights around the very scarce bodies of water.
By Nobu Tamura from https://commons.wikimedia.org Sellosaurus.jpg Licensing cc-by-sa
Plateosaurus were the first herbivores Dinosaurs and was one of the biggest of its time. Plateosaurus traveled in herds and were rivaled by no other. Their biggest enemy was the Starkosaurus which wouldn't have attacked them, but woould have stole their eggs.
By DiBgd from https://commons.wikimedia.org 800px-Dimetrodon_milleri.jpg Licensing cc-by-sa
The not actually a Dinosaur a sauru, the Dimetrodon, was the strongest before dinosaurs were around. They were huge and nothing, but other Dimetrodon, even stood a chance. They could have taken down any prey they wanted. Their huge sail on their back was used for heat regulation, similar to the Stegosaurus.