Short answers, pulled from the story.
Six senators including Padmé Amidala, Mon Mothma, and Bail Organa met in nineteen before the Battle of Yavin to form the initial group. They agreed to a pact of silence while most colleagues disappeared from public service during the Clone Wars.
The Rebel Alliance conducted covert operations using stateless strategy combined with wolfpack-guerrilla warfare against the Imperial fleet. Their military consisted largely of mothballed, improvised, repurposed, or stolen civilian ships because capital ships were logistically expensive to build and maintain.
Cassian Andor stole eighty million credits during the heist of the Imperial payroll vault on Aldhani in five before the Battle of Yavin. This theft created a massive Imperial overreaction that increased security spending and draconian law enforcement policies across the sector.
The Alliance ordered Saw Gerrera assassinated for his war crimes after he launched successful raids against Imperial facilities without formally joining the organization. His ideology viewed anyone helping the Imperial war machine as a legitimate target regardless of civilian casualties which damaged the Alliance politically.
Luke Skywalker successfully guided torpedoes down an exhaust port leading directly to the station's reactor core after intelligence revealed a design flaw. The Death Star obliterated itself in a catastrophic explosion ending the battle in decisive rebel victory despite heavy losses among fighters.