The heirarchy, as I understand it
1) Snoke: Loose analogy - the Emperor
2) Kylo Ren and General Hux: Loose analogy - Darth Vader and Moff Tarkin
3) Captain Phasma: Loose analogy - Boba Fett
Hux apparently does the Tarkin thing, hanging around Starkiller base, ordering people around and making plans that blow up in his face. Snoke sits back and waits for Hux to tell him what's up, while also talking to Kylo about Force bullshit. Sometimes Kylo will go out and get his hands dirty on his own because he's that kind of dude, but it appears that Phasma is more or less the First Order enforcer. She'll show up, look bad ass, and hopefully, as opposed to Fett, actually DO SOMETHING worth the attention.