How can i make my hair shiny,smooth, and straight?

i have curly hair, not extreme curls but enough so it poofs out. i straighten it alot with my chi straightener . I do a good job on it and its not like i miss pieces but when i straigheten it , it looks dull and thick, when my sister straightens her hair its shiny and smooth. Some of my friends also striaghten their hair and its shiny and smooth. How can i do this to my hair ?