OS X Mavericks is available in the Mac App Store now, but it's only an upgrade—but if you want to do a clean install, you'll need to manually burn an installation flash drive. Here's how to do it.
How to create a bootable USB to install OS X Your email has been sent Creating a USB Installer for Apple OS X 10.7-10.8 Creating a USB Installer for Apple OS X 10.5 ...