The Short Version...

Audio book:

• Record yourself reading your book.
• Make it into a series of MP3 files.
• Publish it on ACX or somewhere similar.
• Wait for the cash to roll in.

Really?

The longer version ...

To make an audio book you have to do a few things:

• The first thing you are going to find is that the straw mic on your gaming headset is probably not the right quality for an audio book. That means you have to find a microphone that works for your setup.
• You are going to need pretty decent recording software. Then you have to learn how to use it. Take it slow, there's a lot to process.
• When you have the microphone and you have made your first recording, you you will find out just how noisy your workspace is. PC fans, the dish washer, the postman, passing traffic, drunks walking by, next door mowing the lawn, the house across the road getting in a concrete mixer for a few days. These are all things you have to find a way round.
• Next thing on the list is the room itself. It echoes like an echo chamber. You will need to learn how to damp down the space but not to kill it, nobody likes that dead box sound.
• OK Now you're off to the races. You record one file per chapter and check that the quality is up to scratch, not too loud, not too much background hiss, not to soft.
• Submit it to ACX (or wherever you've decided to use). You're done.

You know What, You need to know How

I will provide a list of external references here, it represents pretty much the best that I've found on a budget so far.

The microphone. There are some good USB microphones, read the reviews. One possible shortcut is a lavalier microphone, they are cheap and the quality can be much better than you expect.

One of my first quality sources on all things to do with recording spoken word is:
Booth Junkie, Mike Delgardio.
He has a youtube channel and although a lot of the stuff is quite old now it's pretty relevant and he's good at explaining his craft. He has quite a lot of equipment reviews, and DIY advice on room treatment and importantly advice on how to get the best out of your microphone and equipment. He also covers a lot about the business and doing audio books.

Audacity is a top class audio recorder, it's available for Windows, Mac and Linux. There's a pretty good free course on how to use it for ACX audio work:
Mike Teaches Audacity
This is a different Mike to the Booth Junkie. He explains it well, shows you where to download the software, and he even shows you how to automate your audio post processing to save time. He demonstrates the Declicker, which is really useful. He demonstrates and describes how to make sure your quality passes the initial ACX checking into the bargain.

Voice Over Angela
Angela is another source of information and she does seem to do a lot more about audio books. She covers a lot that Booth junkie covers but from a slightly different perspective. 

These three sources will give you a good start. The audacity course takes a couple of hours to go through and will give you a good grounding in audio post processing.

Booth Junkie and Voice Over Angela are a bit wider in scope. But they have a lot of experience in the voice over genre, from his room preparation and microphone handling to making it a business.

You have probably got the idea now that you need sot search for voice over as a general topic, possibly voice acting. If you start watching Booth Junkie and Angela you will find similar sources come up in your feed.

Remeber: there is no "best" in this game, you're trying to find what works for you.

When you've got the basics, do some recording. Listen to it and learn.

Have fun while your are learning.