There are 4 main factions competing to control Phoenix :
In order to survive and be able to participate into the conquest of the city, you'll have to join one of those factions. After the Tutorial, you'll have the occasion to discuss with all of them and make your choice. You also have the option to stay in Sewer Rats.
Factions are living entities of the Wasteland. They're composed by a vast number of NPC's, including their leaders, important figures, merchants, assistants and guards, but most importantly by players, who will be proposed to play a major role in their evolution and impact their story and future.
All factions currently provide the same features and opportunities, though with time, they'll become more and more different. They also have a different background and personality, as well as different enemies and priorities.
Every faction has an intimate enemy: Lawyers are the enemy of the Family, and Brahminboys are the enemy of V-Tecs. The hatred towards each other prevents intimate enemies to collaborate at all, and acting against your enemy will be extra rewarded by your faction. However, nothing prevents neutral factions to collaborate, if they wish/need to.
Joining a faction mainly unlocks access to their headquarters, a dedicated safe location providing multiple opportunities. It will also affect your reputation and of course, people with who you'll play.
If you're decided to leave the faction, you need to talk to your HQ base manager. You could try your faction guides to get the detailed info of his location. There's a dialog line where you state your wish, he will explain some of the details of the process. Be advised, that you will lose your reputation, the room and all items inside, even the platoon base if you're the only one who has access in there. After you've left the faction you will appear in the wasteland directly out of the gates of your faction. To join new faction now, you need to talk to dedicated ambassadors which are located in the inner Sewers.