This guide is organized to try and cover all the major areas or objectives in the game without going into every single detail but at the same time trying to convey all the most important information about each topic. Not every single thing is covered here simply because it would make the guide far too long, and because a lot of the fun in Factorio is experimenting and figuring some things out for yourself. Consider this guide as a reference for when you get stuck, or as an initial starting point so you can go into the game with some knowledge.
The sections of the guide are arranged mostly chronologically in the order that that you will get to them in the game. So as you would guess, as you get farther into the guide it becomes more advanced and would require a higher level of experience in order to understand what is being shown and talked about. If you read from the beginning though, you shouldn't really have a problem understanding what is going on in the later sections. Each section is linked in the Table of Contents, so if you click on it in the ToC, it will jump to the page where that section starts, so it should be easier to find what you're looking for.
For information about specific items you can look at the Item Index which lists every item in the game followed by a short description of what it is or does. The Item Index is arranged exactly how it is shown in the game to make it easier to find things.
If you are looking for setups and builds for certain items, such as some good Circuit Builds or more Train unloading/loading stations you can check the Additional Information, Setups & Builds section.
So, let's begin and remember the main objective of Factorio is to have fun!