Sam was the first of the boys from the reservation to phase and become a werewolf. The first time he met Bella, he said that the Cullens were not welcome at First Beach, which sparked the conversation leading Jacob to tell Bella about vampires and werewolves.
As the other boys on the reservation became werewolves one by one, Sam took on the role of Alpha and led them through their change and helped them become accustomed to their new life. To outsiders, it seemed as if he had started a cult. When Jacob became a werewolf, it was Sam who gave him the injunction (an absolute command given by the Alpha) not to tell Bella. Once Bella figured out the truth, Sam accepted her help with the search for Victoria.
In Breaking Dawn when Jacob tells the pack that Bella is having a baby, Sam declares that they have to kill the fetus, and Bella along with it, as a necessary casualty to protect the tribe. Jacob embraces the Alpha heritage in his blood and he, Seth, and Leah leave the pack so they can protect the Cullens. Sam concluded that if Bella was killed, Jacob would be so mad that he would lead both packs in attacking the child. This never happened: Jacob wanted to kill Renesmee, but imprinted on her once he saw her. Sam and his pack were left with no choice but to declare true peace with the Cullens.
Sam and his pack later stood by the Cullens when the Volturi confronted them. After the incident, Sam and his pack returned to La Push.