Warning: The following contains spoilers for the Phasma novel.
The Force Awakens introduced audiences to a new era of Star Wars, including a bevy of new characters of varying allegiances, from Rey to Kylo Ren. There were a few moments of peril and destruction, most notably killing off Han Solo, but outside of a few Resistance pilots (and everyone in the Hosnian System), all of the new characters made it out alive. With everyone set to return for The Last Jedi, almost everyone is also carrying an element of the plot or backstory forward, except for one: Gwendoline Christie's Captain Phasma.
Despite her awesome chrome stormtrooper armor and intimidating demeanor, almost nothing is known about Phasma, partially because she has very little screen time to establish any kind of backstory. That all changes with Phasma by Delilah Dawson, a new novel in the Journey to The Last Jedi series of tie-in stories that tells most of the First Republic Captain's life story leading up to within a few years of The Force Awakens.
While Phasma is fairly thorough, it's important to point out that the story is being told by a questionable narrator - a captured Resistance spy name Vi Moradi who got all of her information from an old friend of Phasma's, Siv. Per Vi's internal monologue, the story is entirely accurate, aside from some slight embellishment, but all her information is second hand from Siv. So, while this bit of Star Wars canon is fairly well documented, there's always a good chance that things didn't play out exactly as described. Even so, it was probably pretty close.
The planet Parnassos was once lush and green, teeming with all manner of wild species and ripe for farming - until the arrival of Con Star Mining. Arriving as many as several hundred years before the events of the Star Wars movies, Con Star's Parnassos endeavors were intended to improve the lives of their employees and those that lived on the planet, but some combination of shoddy work and lax regulation resulted in a nuclear explosion at one of their cities, destroying all life within the blast radius and leaving a nuclear fallout that would slowly expand across more of the planet, poisoning the air and killing animal and plant life.
Con Star abandoned Parnassos as all the resources were useless. Some of their employees ended their own lives, leaving behind teams of droids to mindlessly continue running any functional stations. Other inhabitants, whether natives, Con Star employees, or some combination of the two lived on, forming tribes and living in primitive isolation until the memory and understanding of advanced technology was little more than legend and the daily struggle for survival occupied most of their waking lives. This is the Parnassos into which Phasma was born.
Phasma's parents and her brother Keldo lived with a small tribe in a cave they called the Nautilus. When she was a young girl, the already scarce sources of food slowly dried up, wounds - even minor ones - would fester and lead to life-threatening infection, and people's hair began to fall out. Since the Nautilus was one of the few safe havens in the area, other tribes, the Claw and the Scyre, wanted it for themselves. Phasma's tribe would routinely have to fight off attacks from both the Claw and the Scyre, maintaining control of the Nautilus, but slowly decreasing their already small numbers.
Phasma urged her family to unite with the Scyre to form a larger group that could continue to fight off the Claw, but they refused. Eventually, behind their back, Phasma made a secret agreement with the Scyre. One night, she attacked her brother Keldo, stabbing him in the leg and pushing him into the Nautilus, and when he woke up, he and Phasma were the only survivors, and the Scyre had taken over. His leg had to be amputated to avoid infection, but the Scyre had a special oracle salve - harvested from the bodies of their fallen in a sort of pseudo-religious ceremony using devices called "detraxors" - that could be applied to fresh wounds to allow it to heal properly. Keldo and Phasma only needed to pledge their loyalty to the Scyre, which they did.
As a member of the Scyre, Phasma grew into a mighty warrior, both in size - she was one of the tallest people in the Scyre - and skill, regularly leading her own elite warriors in combat against the Claw or on scavenging missions. Keldo, too, became a significant part of the Scyre. While he was missing his leg and not as useful for combat or manual labor, he was one of the few - if not the only - member of the Scyre that could read, and eventually found himself sharing Scyre leadership with Phasma.