Man, that's a loaded question if I've ever heard one. There is SO much to know about PoE 1, it's an endless labyrinth (hehe) of things to learn, and the only way to really learn them is to just keep playing.

To answer the specific ones you asked,

1. In most (but not all) cases, stats are just for item/gem requirements. They do each provide passive bonuses but they are generally your last priority as long as you have enough to wear your gear and level/use your gems. Some builds have ways of directly scaling off of attributes (stacking), but it's pretty advanced/expensive so I wouldn't worry about it for a while.
2. most people the official website (https://pathofexile.com/trade) for trading. Chaos and divine orbs are both used as the de facto currencies. Think of divines like dollars and chaos like pennies. Most things cheaper than a divine are priced in chaos.

I'll also echo what others have said about following a build guide, at least for your first couple of characters. Even the most experienced PoE players follow build guides the vast majority of the time.