OS X Mavericks is available in the Mac App Store now, but it's only an upgrade . If you want to do a clean install, you'll need to manually burn an installation flash drive. Here's how to do it.
- Insert your USB drive (you'll need one that's 8GB or larger) and open Disk Utility.
- Select your drive in the sidebar and go to the Erase tab. Format the drive as "Mac OS Extended (Journaled)" and name the drive "Untitled".
- Click the Erase button and wait for Disk Utility to finish.
- Close Disk Utility and open up a Terminal window. Type the following command into the Terminal:
- Press Enter. The process should take about 20 minutes, so don't cancel it or eject your USB drive while it's doing its thing.
- When it's done, you should see a message stating the process is finished. Now, you can restart your computer, hold the Option key to access the boot menu, and select your new USB drive to install a clean copy of OS X Mavericks.
How to make a bootable install of Mavericks on Flash USB Drive? [MacRumors Forums]