"How much do I need to know about Fate? Is it alright if I'm a newbie?" It is perfectly alright for you to be new to Fate, and it is always a pleasure to show new players the elegance of this system; be my guest.
"What exactly does an 'Avatar of the Dark God' look like?" Anything you want! Well, anything preferably humanoid, at least. The creations of the Dark God come in many forms, so as long as it seems demonic in some capacity, it should be good.
"My Prince/ess of Darkness has only just been 'born'; how do we differentiate our applications from one another?" Well, speaking in the meta, there's a certain reason that the personality and character of this entity could be wildly different from fate to fate (heh). Additionally, the story will begin at least a few weeks or months from their initial summoning, so your character will have had the time to have some interesting encounters if that's what you need. We could also say that the Dark God took your soul - unsummoned - to It's realm, first, and that you were able to learn many things from your demonic subjects and watch the material world from the comfort and safety of your hell-palace; it gets a little bit wonky because the Demon Realm is not entirely physical (it's partially conceptual) and the Dark God has no true form Itself (meaning no playing catch or getting hugs), but you might be able to do something with that.
"What are the 'forces of good'?" That term is a catch-all for hero and paladin types; it could very simply be the acolyte of some religion of holy warriors who banish the darkness with the power of light, or a champion of good on a path of righteousness and honor. It could also be something more niche/specific, I don't know. If you go with a religious type, you should save a space in your stunt list for something like Nihlia's Holy Power ability.
