The Force Awakens, the seventh Star Wars movie, focuses on a young woman, Rey, as she becomes entwined with efforts - led by General Leia, no longer a princess - to locate a vanished Luke Skywalker and generally save the galaxy from evil combatants called the First Order. The Classic Trilogy featuring A New Hope, The Empire Strikes Back and Return of the Jedi.
Disney now owns Star Wars but has been keeping the streaming rights close to the vest. There isn't a legitimate service yet that has access to the original Star Wars Trilogy streaming online. Star Wars Rebels is available to stream on DisneyXD's Site and App. Or you can purchase the Star Wars DVDs or Blu-Rays to eliminate the need for streaming. Purchase this set from Amazon in a bundle for under a hundred bucks or so. You can get all six films in high definition in the "Digital Movie Collection" from Google Play Movies and Amazon Movies, or on iTunes.
Or you can do a bit of legwork to download and watch it. You can download the Despecialized Edition from TehPARADOX.com. By far the easiest way to obtain the previous movie is through BitTorrent. Just visit your favorite BitTorrent search engine (like torrentz.eu) and search for "star wars despecialized edition". You'll probably find quite a few options. Remember to use a VPN to keep yourself anonymous when you download.