Were Did Dinosaurs Come From?

First, you must understand the Theory of Evolution. This is a process that can take 1000's of years to even develop a small change in plants and animals. But to your question. Dinosaurs evolved from smaller animals. Their environment contributed to their size and growth. Modern day example: A giraffe originally looked like a horse and developed it's long neck to reach the upper branches of the tree leaves it feeds on. Same with dinosaurs. They adapted to their surroundings to become more efficient at survival, which often resulted in bigger animals. Hard question to answer in one paragraph, but I hope you get the idea.