Notorious smuggler and captain of the Millennium Falcon, Han Solo agreed to transport an old man named Ben Kenobi, a boy named Luke Skywalker and two droids named C-3PO and R2-D2 to Alderaan. He needed the cash to pay off Jabba the Hutt, who just put a price on his head for botching a smuggling job.
Unfortunately Alderaan got blown up by the Death Star. After he helped rescue Princess Leia from the Empire, he wanted to leave and get far away from the coming battle. But then he had a change of heart, saved Luke so that he could blow up the Death Star, and ultimately joined the Rebel Alliance.
And the rest, as they all say, is history.