Like many people said, the best way to go about rules is to have the SRD handy and check the necessary rules whenever they come up - you don't really need to know them by memory and you'll soon learn the ones you need more often.
On the matter of the setting, improvising is likely less hard than you think - get yourself a few dozen fantasy maps, cobble them together and rename/repopulate as necessary to fit your player choices and concepts. Using real-life cultures as models is also pretty easy, particularly the less known time periods - everybody has an idea of what happened in France during the Revolution, but the time period after the Restauration is much less well known, despite being the choice setting for a number of famous fiction. As for gods, the SRD offers stats for a multitude of them - here's the link, just pick the ones you like the most, or which fit your setting choices the best.
http://www.d20pfsrd.com/classe...ains/gods-pantheons/
That said, if you're using Pathfinder as a system, why not use the in-house setting of Paizo, Golarion? It has a perfectly free accessible wiki, a setting that allows for pratically every plotline or ideas you could possibly want to explore, and offers a wealth of fluff for many classes, seeing as they were created with the setting in mind. I can understand not wanting to use the modules/adventure path if you have a story of your own you want to use (even if, as said, a lot of them are really good) but the setting itself can be used indipendently.
As for those rules who seem strange, that's because a good number were inherited from D&D 3.5, which itself was adapting an ever older system. Sorcerer use Charisma to cast because the amount of power they control is supposed to depend on the force of their personality, which is what Charisma controls; and hits depend on Strenght on the simple principle that stronger characters are better at close combat (unless special training is involved, which is what feats are supposed to represent).
If you were asking from a mechanical standpoint, though, STR determines to hit because otherwise it'd be fully useless (since it only determines melee to-hit and damage, and the two most useless skills; compare with DEX, which is the strongest stat in the game as it determines a whole ton of things). In fact people drop it enough already in character creation as it is, since most casters don't need it and archery is superior to melee anyway.
About the Sorcerer, the truth is that it was designed to be weaker than the Wizard. Thus, since casters become deadlier when they can maximize their casting stat, they were assigned CHA because it is the most useless stat in the game, as it only determines the effect of a few skills. Most of the other spontaneous casters simply followwed the Sorcerer because, ironically, it's more balanced: spellcasting is the most powerful ability in the game, so tying it to the weakers stat makes it slightly harder for casters to dominate everything, at least at the lower levels and/or with lower point-buy options.
This message was last edited by the user at 21:44, Thu 20 Apr 2017.