The old EU places the formal creation of the Alliance to Restore the Republic at two years before the Battle of Yavin (2 B.Y.) with the signing of the Corellian Treaty. This was basically the merger of several rebel factions into a larger orginized force better able to handle the Galactic Empire.
There has been rebellion or sparks of it since just before the Republic fell. The sparks were the push by some 2000 Senators to get Palpatine to return his emergency powers to the Senate as the Clone Wars ended, and the Jedi's failed mission to arrest him as a Sith Lord. In the 19 years between at and the Battle of Yavin there were rebellions and the remaining Jedi that were both hunted by the Empire.
However at somepoint, not long before the Battle of Yavin, the new Alliance won a major victory against the Empire, and as a result gained the plans for the Death Star.
Star Wars: Rebels takes place starting five years before the Battle of Yavin. Princess Leia Organa is 14 years old and may become an Imperial Senator soonish (if she isn't already, given that her real mother was an elected queen of a planet at age 14). The cast lists Billy Dee Williams, probably as Lando, so We will likely see him, probably while he still owns the Millenium Falcon. We might also see his droid partner Vuffi Raa.