How did heidi from the hills get discovered?

She wasn't exactly "discovered". Heidi and Lauren Conrad became friends when they both attended design school in San Diego. After Lauren left San Diego, Heidi came to spend the summer with with her and subsequently was featured (mostly in the background) in a few scenes of Laguna Beach season 2. She models in the 'Fight the slide' episode (with Kristin Cavallari no less!) and you can also see her twice during the episode where Lauren goes on her first date with Jason.