To get it to show up, you will have to install an update from this page. After rebooting, the Store will show the preview. From there you can install the preview; a 2GB download and a couple of reboots later and it will be done.
As with any beta software, this is probably not something you want to do on a production machine. Microsoft says that while some people will be able to uninstall the preview (and then upgrade to the final 8.1 build when it’s available), under some circumstances this isn’t possible, so you’ll have to be prepared to blow the machine away to upgrade it.
If you’re not running Windows 8, you obviously won’t be able to install from the Windows Store. Microsoft will make ISOs available to download some time over the next day.