You can always read these guides in any order, but here's a quick rundown of what each is for:

I'm New! Halp!! (Newbie Guide)
Recommended for all. This is probably where you should start. This talks about what Post Terminus is, what to expect, and how to get started. Even if you RP'd previously in Halcyon Days, I'd still recommend running through this one.

Post Terminus FAQ
Recommended for people who used to be part of the Halcyon Days Bleach RP. It covers some of what's the same and what's different in PT.

The Post Terminus Mega-Thread
Recommended for reference as needed, but not required reading to get started. Much longer and more detailed than the I'm New! guide, this thread is more about the world of PT, giving information on its people, its culture, its organizations, and the skills. It's a good all-in-one reference to come back to, but it may be a lot to read through all at once.

Crafting Guide
Recommended for crafters, optional for others. Crafting is still pretty free-form, but this gives an idea of a great many of the things that can be created, and what is involved in terms of materials and time. If you plan to be a smith or an engineer, I would recommend this, but otherwise, you can safely skip this one.

Moot's Super Guide to Being a Better RPer
Entirely optional. This guide is very old, written by a previous GM named Mootiman. Although it's tailored for our old Bleach RP, a lot of the lessons are still applicable in other settings. It can be harsh at points, but it's well worth a read.