Which came first? The constellation or the gigantic hunter? Well, it totally depends on whom you ask. Some scholars think that the ancient Greeks looked up into the sky and said, "Wow, that group of bright stars looks a whole lot like a giant holding a huge club. Let's call him Orion and make up stories about him. Yaaayyyy!"
Other scholars think those stargazing Greeks said, "Man, that group of stars looks a whole lot like Orion, who we just so happen to have tons of stories about already. Let's name those stars after him and incorporate them into our oral tradition. Yaaayyyy!" There's really no way to know which way it went down, because these days the mythological giant and the constellation named after him both burn just as bright.
Unlike a lot of other famous Greek heroes, Orion doesn't have any big long definitive text that lays out his whole story. Maybe, there was one way back in the day, but if there was, somebody lost it. (Nice going, somebody!) So, these days we've got to piece together Orion's story from a bunch of different sources, most of which don't really agree with each other. You can find mentions of Orion by writers like Homer, Hesiod, Hyginius, and Pseudo-Apollodorus among others.
Orion is still waiting for the big Hollywood blockbuster made about his life, but he does appear in two operas by everybody's favorite modern composer, Philip Glass: Galileo Galilei, Orion. (Wow, artsy.) Oh, and the big guy also made a cameo on the Disney animated TV series about the life of Hercules. (Wow, not so artsy.) It's okay, Orion, you don't have to wait for Hollywood to make you a star. You already are one. Well, you're actually several stars, plus a nebula, but hey, who's counting?