First of all, if you need any help getting your system configured to start developing Windows 8, just go to usdpe.ohours.org and set up an office hours appointment with a Microsoft Developer Evangelist near you. They'll walk you through the whole process.
Windows 8 development is not only a blast, but it can be very lucrative as well. I encourage you to dive in and work hard at learning. If you need any help along the way we'll be glad to help. Here's how to get started...
Get a Windows Store developer account
To put an app in the Windows Store (and start making money!!), you need to get a Windows Store developer account. The account costs money on an annual basis, but if you qualify for one of the programs I'm about to mention, you could get it for free.
If you're a student, you can get a Windows Store developer account for free by joining the DreamSpark program! Just go to http://www.dreamspark.com and sign up. If you need any help getting signed up for DreamSpark, email my colleague Sam Stokes at sam.stokes at microsoft.com from your school email account. He's an expert at that stuff.
If you are a new business with a BizSpark account or if you have an MSDN subscription, you too can get your Windows Store developer account for free. For BizSpark questions, email my colleague steve.seow at microsoft.com and he'll get you squared away.
If you don't qualify for any of these programs, then you'll need to pay for your developer account. They are $49 for private parties or $99 for companies. You can go here to get started.
For more in-depth information about the developer account visit Registering for a Windows Store developer account in the Dev Center.
Get Windows 8
To develop a Windows 8 app, you need to get Windows 8 and install it. Actually, if you are using some of the game engines, you could start developing before you get Windows 8, but eventually you'll need it for testing and submitting your finished product to the Windows Store so you can start making the big bucks.
You can buy Windows 8 online or in any computer store. If you qualify for one of the programs I mentioned (DreamSpark, BizSpark, or MSDN) then you can get a free license. If you want to try Windows 8 out for a while, you can always download a 90-day evaluation copy.
If you have a PC, you can install Windows 8 natively (recommended), you can boot to a VHD drive, or you can install it in a virtual machine. Install it natively if you can or boot to a VHD drive as a second choice. If you run it in a virtual machine, you might not get the same performance.
If you have a Mac, you can install Windows 8 using BootCamp (recommended) or virtually using Parallels (also good). There are other virtualization options, but these two are recommended. Note that Apple says that BootCamp doesn't support Windows 8, but I've seen it work, so I think you'll be okay.
Get the developer tools
Besides Windows 8, you also need to install the tools. At a minimum, you'll need the free Visual Studio Express 2012 for Windows 8. There is a professional version of Visual Studio 2012 too that costs money but offers more features. The free version is enough to build an entire app, though. If you want to use a game engine for making a game for Windows 8, let me recommend Scirra Construct2. Contruct2 is an excellent tool and can really get you from 0-60 in no time flat!
There are a few other SDKs (for maps or advertisements or whatever) that you can download from the Windows Dev Center.
Get a graphics program
Finally, whether you're creating a business app or a video game, you're going to need to generate some graphics. You can use the graphics package of your choice, but if you don't have any experience with graphics yet, I recommend CorelDRAW. CorelDRAW is a vector-based graphics package, which makes it much easier to create new illustrations and app graphics. CorelDRAW costs money, so if you are looking for a free solution, then InkScape will get the job done too.
Are you stuck?! Don't forget you can set up office hours with a Microsoft Developer Evangelist for one-on-one help by visiting usdpe.ohours.org.
Beyond getting your machine all set up, you can try these links to get you entrenched in Windows 8 development...
Download some sample code
Install the codeSHOW app