I'm pretty used to TTRPGing with a d20 system and I can play a d10 system with a good GM.

Dark Heresy REALLY interests me and I've gotten my hands on all the books as a late birthday present.

I want to GM a Dark Heresy campaign and I'm not really sure where to start to get to know the game mechanics and whatnot.

Which book would be best to start with, and which books should I read in what order?

Well of course read the DH core rulebook first. You can skip the "what is a RPG" part, but you should get to know the basic rules and fluff at least, then a little about the combat rules and character creation rules. Then the GM chapter, especially the "life in the imperium" part. Decide what kind of campaign you want and only then check out other books such as the Inquisitors Handbook, Creatures Anathema etc.

I found it a good idea to play a test scenario first with your players, so they get a feel for the system and setting. Play something that can be finished in a session or two, have pregenerated characters. Some of my players fell in love with certain arcehtypes (the scum and psyker), the others picked different classes.

Have fun!

Agreed, the core DH rulebook is best to start.

But for added color, especially if you're not very familiar with the DH universe (from your post, I wasn't sure if you had ever stepped into th 40K universe before), try reading the book 'Eisenhorn', as this gives a very good feel for the universe setting.

I personally got these books in this order.

1. GM Screen. Contains an introductory adventure and a cool system to create your own critters and aliens.

2. Inquisitors Handbook. Expands character options and weapons selection.

3. Creature Anathema. Even more creatures with cool adventure hooks for each one.

If I wanted to keep it real simple and cheap, I would say the GM Screen gave me the most bang for the buck.

The creature creation rules work for animals and intelligent races and I have even let the players in some games play aliens or mutants using this stuff.

Of course that is completely house ruled and non-official, but plenty of fun nonetheless.

Of course start by reading the core book, cover to cover. All the other books are "extra" rules that you don't need to get going, and you should leave off until you have a good grasp of the basics.

Run a canned adventure, with pre-made acolytes for your players. This means they don't have to worry about anything but learning the rules, and you get a good look at how characters go together by making them. And a canned adventure has been playtested and ordered, keeping you, the new GM, from doing anything other than setting the scene and learning the rules with your players. I'd reccommend "Illumination" from the core book. It's only a few sessions long, can generate lots of hooks and memorable NPC's to use later on, and has a good mix of elements. There's diplomacy, investigation, mystery, horror, and of course, bloody dangerous combat. In short, everything that makes DH such a good game. Good luck!