Shepard was born on Earth. His mom died in the slums, of an overdose. His father died during the First Contact War with the Turians. He ran with a rough crowd, till one day he enlisted in the Alliance Military. It was there that he found purpose, he found a family. His dedication to The Alliance strengthened to the point of landing him in the N7 program. His biggest accomplishment at the time came during a mission on Torfan where his team fought overwhelming numbers in order to get the pirates hiding there. All but Shepard perished, but the mission was a success and he was awarded for making the tough calls. Eventually he found himself under the tutelage of David Anderson. A decorated hero in his own right, Anderson had once been a candidate for humanity's first spectre. Unfortunately, the role never went to the career military man. Yet it was received by his protege, when Turian Spectre Saren went rogue.
Shepard chased Saren across the galaxy in the Normandy SR-1. A joint project of Turian and Human collaboration, it was the fastest vessel in The Alliance fleet. Shepard was spotted on Noveria helping root out the corrupt officials stationed there. There were also accounts of him appearing to aid the colonists of Feros, and Terra Nova. During these adventures he collected a handful of individuals that would become important in the coming years. The quarian Tali'Zorah Vas Normandy (Nar'Raya at the time), the turian C-Sec officer Garrus Vakarian, Lt. Ashley Williams, Lt. Kaidan Alenko, the krogan mercenary Urdnot Wrex, and Liara T'Soni all aided Shepard in his pursuit. Things came to a head on Virmire, Saren had set up a cloning facility for krogan with the supposed intention of "curing" the Genophage. This brought Shepard into contention with his krogan friend Wrex, but fortunately the two were able to come to an understanding. Lt Williams informed me that'd she'd been ready to deal with Wrex should the need have occurred. Shepard destroyed the facility, but the resulting explosion took the life of Lt Alenko in the process. Some I talked to believed it was due to Shepard's burgeoning affection for Lt Williams at the time that fueled his choice to leave Alenko behind. I only know it was something that weighed heavily on both the Commander and Williams even in the short time I spent with them during the Reaper War.
Much has been written about Donnel Udina and the previous council's grounding of the Normandy. Some have called them cowards, others believe they were under the influence of The Reapers even then. Whatever their reasons it matters little, for the Normandy raced towards Ilos to confront Saren. It was here the crew first encountered what remained of the Protheans there. Shepard and company gave chase to Saren, ending up at the Citadel.
Shepard confronted Saren and managed to convince the Spectre to take his own life rather than become a slave to Sovereign and The Reapers. Unfortunately, it was too late, Sovereign (the Reaper Saren had been working within) took control of his lifeless husk to stop Shepard. The Citadel fleet, alongside members of the Alliance navy hit Sovereign with all they had, even keeping up the barrage to the point of sacrificing the council. In the end Shepard prevailed, and Captain Anderson was appointed as representative to the new council.