There are 2 ways to build Office 2013 and SharePoint 2013 apps. We can use Office 365 online tools or we can use Visual Studio 2012.
I plan to use Visual Studio 2012.
I just downloaded and installed Microsoft Office Tools for Visual Studio 2012
on Windows 8 machine. You need this to build Office 2013 and SharePoint 2013 apps using Visual Studio 2012.
Once successful installation, when you create a new project, you should see 2 new project templates - App for Office 2013 and App for SharePoint 2013. These templates are used to build Office and SharePoint apps.